maintenance solutions for bleeding and flushed pavements

Transcription

TEXAS DEPARTMENT
OF TRANSPORTATION
MAINTENANCE SOLUTIONS
FOR BLEEDING AND
FLUSHED PAVEMENTS
RESEARCH PRODUCT 0-5230-P1
RESEARCH STUDY NO. 0-5230
www.techmrt.ttu.edu/reports
NOVEMBER 2006
MAINTENANCE SOLUTIONS FOR BLEEDING AND FLUSHED PAVEMENTS
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TEXAS DEPARTMENT OF TRANSPORTATION
by William D. Lawson, P.E., Ph.D.
Senior Research Associate
TECHMRT: Multidisciplinary Research in Transportation
Research Product 0-5230-P1
Research Study No. 0-5230
study title
Short Term Solutions to “Bleeding” Asphalt Pavements
sponsored by
Texas Department of Transportation
in cooperation with
U.S. Department of Transportation
Federal Highway Administration
project director
Terry Paholek, P.E.
program coordinator
Joe Graff, P.E.
project advisory committee
Michael Heise, P.E.
Lewis Nowlin, P.E.
Charles Russell, P.E.
Mark Shafer, P.E.
November 2006
TECHMRT: Multidisciplinary Research in Transportation
TEXAS TECH UNIVERSITY
Box 40123
Lubbock, TX 79409-1023
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DISCLAIMER
The contents of this report reflect the views of the authors who are responsible for the facts and the accuracy of the data presented herein. The contents
do not necessarily reflect the official views or policies of the Texas Department of Transportation or the Federal Highway Administration. This report
does not constitute a standard, specification, or regulation, nor is it intended
for construction, bidding, or permit purposes.
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PREFACE
This document presents recommended maintenance solutions for treatment
of bleeding and flushed pavements with a seal coat or a surface treatment.
The following topics are covered:
•
Maintenance definition of bleeding and flushing
•
Factors that contribute to bleeding and flushing
•
Preventive maintenance considerations
•
Maintenance solutions for bleeding pavement surfaces
•
Maintenance solutions for flushed pavement surfaces
•
Treatment of bleeding and flushing at intersections
Expressed within the broader context of seal coat design and construction,
the maintenance solutions described herein are intended to help TxDOT
maintenance forces address the problem of flushed and bleeding pavements
on Texas roadways in order to better provide for the safe, effective and efficient movement of people and goods.
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TABLE OF CONTENTS
PREFACE .......................................................................................................................... 5
INTRODUCTION ................................................................................................................ 8
DEFINITIONS ..................................................................................................................10
Maintenance Definition for Bleeding Pavements...........................................................10
Maintenance Definition for Flushed Pavements ............................................................12
CAUSES ..........................................................................................................................14
Aggregate Issues ...............................................................................................................16
Binder Issues .....................................................................................................................18
Traffic Issues .....................................................................................................................20
Environmental Issues .......................................................................................................22
Construction Issues...........................................................................................................24
PREVENTIVE MAINTENANCE .........................................................................................26
Potentially avoidable causes............................................................................................28
Centralized seal coat program management .................................................................30
Binder selection.................................................................................................................34
BLEEDING.......................................................................................................................36
Apply layer of small size (grade 5) aggregate .................................................................38
Apply layer of larger size (grade 4 or 3) aggregate.........................................................42
Apply blotter material to blot up excess asphalt (coarse sand/stone screenings) .....46
Sandwich seal....................................................................................................................50
Apply lime water to cool and crust over bleeding...........................................................54
Apply water to cool pavement surface ............................................................................62
Remove bleeding pavement surface and replace with new seal coat .........................64
Other solutions ..................................................................................................................66
FLUSHING .......................................................................................................... 68
Cold milling to remove bleeding asphalt with/without replacement ............................70
New seal coat ....................................................................................................................74
Microsurfacing...................................................................................................................76
Thin asphaltic concrete overlay .......................................................................................80
Ultra high pressure watercutting to remove excess asphalt and restore texture........84
Other solutions .................................................................................................. 90
INTERSECTIONS.............................................................................................................92
Microsurface......................................................................................................................94
Hot mix asphalt pavement ...............................................................................................96
Portland cement concrete ................................................................................................98
Racked-in Seal................................................................................................................ 100
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INTRODUCTION
RESEARCH CONTEXT
This report summarizes the findings of research sponsored by the Texas Department of Transportation (TxDOT) directed at identifying solutions to the
problem of bleeding and flushed asphalt pavements with seal coats or surface treatments.
The research focused on documenting typical manifestations of bleeding and
flushing, discovering the cause(s) and underlying factors that contribute to
bleeding/flushing, and identifying cost-effective treatment approaches.
In addressing these challenges, the research synthesized literature and related information from sources outside TxDOT and also attempted to capture
the wealth of institutional knowledge and expertise resident within the
agency, both at the Division and District levels. The research challenge was
to boil everything down to a manageable number of definable practices for
treating (or preventing) the bleeding/flushing problem.
THE LAYOUT OF THIS REPORT
More than a little confusion exists, both in published academic literature and
in the roadway maintenance community, about the differences between
bleeding versus flushed pavements. One of the more helpful descriptions we
heard in the Districts is that “flushed” is past tense; whereas, “bleeding” is an
active verb. To address confusion about these terms, we begin with a brief
discussion of maintenance definitions for bleeding and flushed pavements.
The next section of this report — on causes — provides insight into the main
factors that contribute to bleeding and flushing. The maintenance supervisor
looking for solutions to these problems will do well to recognize and understand these causes, since in many cases the best solution is to not get into
the problem in the first place.
Treatment of bleeding and flushed pavements is considered corrective as
opposed to preventive maintenance. However, the preventive maintenance
mindset is nevertheless very useful. This is for the simple reason that the
preventive mindset is by far the most cost-effective maintenance solution
discussed herein. “You show me a maintenance leader who approaches seal
coat/surface treatment design and construction from the perspective of how
to achieve high quality and avoid bleeding and flushing, and I’ll show you a
maintenance crew that is relatively unbothered by the bleeding/flushing
problem.”
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RESEARCH CONTEXT AND LAYOUT OF THIS
REPORT
The remainder of this document presents maintenance solutions for three
specific contexts: bleeding pavements, flushed pavements, and intersections. The introduction to each section provides an overview and a broad
comparison of the treatment methods. The discussions of the individual solutions begin with a brief description of the treatment method, followed by a
discussion of its applicability and effectiveness, materials and equipment,
procedures, helpful tips, and concerns.
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DEFINITION:
BLEEDING
As used in this document, bleeding is the upward movement of asphalt in a
seal coat or surface treatment resulting in the formation of a film of asphalt
on the roadway surface. Bleeding occurs when excess asphalt binder fills the
voids in the aggregate mat and then moves upward to the pavement surface
under traffic and with heat expansion in a non-reversible, cumulative process.
VISUAL IDENTIFICATION
Maintenance personnel have described bleeding as the rapid onset of live
(liquid), excess asphalt on the pavement surface which rises above the aggregate. The appearance of the pavement surface goes from dull black to
glossy, shiny, and glass-like.
Traffic through a bleeding pavement makes a “smacking" sound, like "driving
through rain” — sometimes termed “singing.”
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MAINTENANCE DEFINITION FOR BLEEDING
PAVEMENT
WHY BLEEDING IS A PROBLEM
Functionally, bleeding is considered a failure. Bleeding asphalt sticks to aggregate and tires and causes tracking. Left unattended, bleeding can result
in the seal coat or surface treatment aggregate rolling over and being picked
up. This can lead to “pitting out” or pulling up chunks of pavement —
“slinging” as opposed to “singing.” Bleeding typically occurs during seal coat/
surface treatment construction before the binder has fully cured.
MAINTENANCE CONCERNS
Factors that lead to bleeding include but are not limited to binder issues (too
much, wrong type), environmental conditions (temperature, humidity), traffic
effects (heavy traffic, high volume traffic), and construction deficiencies.
Bleeding is an immediate maintenance problem that must be repaired. Repair response is time-critical to prevent picking up a fresh seal coat or surface
treatment.
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DEFINITION:
FLUSHING
Like bleeding, flushing describes a pavement condition where excess asphalt
binder occurs on the roadway surface. Many pavement maintenance definitions use the terms “bleeding” and “flushing” interchangeably; they do not
distinguish between the two. However, for the purposes of this document,
flushing differs from bleeding in certain key ways.
FLUSHED PAVEMENT: NOT NECESSARILY A BAD
THING
First and foremost, flushing is not an immediate maintenance problem and
may not be a maintenance problem at all. In a flushed pavement, asphalt
fills the voids in the aggregate mat and comes up flush with top of rock, but
the binder is not liquid — it is solid or semi-solid. A flushed pavement is still
holding its seal.
Second, flushing occurs on pavements where the asphalt has already cured;
whereas, bleeding typically occurs during the seal coat or surface treatment
construction period while the binder is still tender. In contrast, flushed pave
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MAINTENANCE DEFINITION FOR FLUSHED
PAVEMENT
ments have typically been through at least one full winter/summer season.
Progressive flushing due to wearing down of aggregate is common during the
typical life cycle of a properly-constructed seal coat.
VISUAL IDENTIFICATION
Depending on the degree of severity, flushed areas of the pavement surface
may be discolored/darker (low severity), experience a loss of surface texture
(moderate severity), or have a shiny, glossy appearance showing tire marks in
warm weather (high severity). Flushed pavements can track and bubble,
eventually going to bleeding. The excess asphalt associated with flushing
frequently appears in the wheel paths.
MAINTENANCE CONCERNS
The maintenance threshold for flushed pavements is a slick roadway surface
with low skid resistance. Flushed areas become very slippery, especially in
wet weather conditions, which is a safety concern. The condition becomes
particularly severe when flushing coincides with rutting and water accumulation in the wheel paths.
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CAUSES
This section of this report provides insight into the main factors that contribute to bleeding and flushed pavements. These include aggregate issues,
binder issues, traffic issues, environmental issues, and construction issues.
The maintenance supervisor looking for solutions to bleeding/flushing will do
well to recognize and understand these causes, since in many cases the best
solution is to not get into the problem in the first place.
AGGREGATE ISSUES
When a seal coat or surface treatment loses its rock, flushing and/or bleeding will be the result. Aggregate issues include rock loss, application of too
much rock, use of dirty rock, use of soft rock, and judicious use of modified
aggregate grades.
BINDER ISSUES
The development and appropriate use of polymer modified and other binders
have improved seal coat and surface treatment performance due to their
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FACTORS THAT CAUSE BLEEDING AND
FLUSHED PAVEMENTS
increased cohesion, toughness, and reduced temperature susceptibility.
Binder issues relative to bleeding and flushing include binder selection, historically problematic binders, binder application rate, binder curing, and
binder quality.
TRAFFIC ISSUES
Certain types of traffic can press aggregate into the seal coat/surface treatment matrix or it can result in aggregate being dislodged or rolled over by
turning stresses. Traffic considerations relative to bleeding and flushing include traffic volume, traffic type (heavy trucks), traffic movements (stop & go,
turning, etc.), and intersections.
ENVIRONMENTAL ISSUES
Environmental issues such as temperature and humidity have a profound
effect on flushing and bleeding. This section of this report discusses the key
environmental factors associated with flushing/bleeding pavements including
high temperature, humidity, changing temperatures, and low temperatures.
CONSTRUCTION ISSUES
The dominant causes of flushing/bleeding pavements with seal coats and
surface treatments fall in the category of construction issues. Most of these
causes are preventable with good design and construction practices. Construction issues relative to preventing/avoiding bleeding and flushed pavements include proper assessment of existing pavement conditions, use of
good seal coat preparation techniques, treatment of rutting in wheel paths,
attention to the special curing needs of new seal coats, use of fog seals to
mitigate rock loss problems, and avoidance of poor construction practices.
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AGGREGATE ISSUES
ROCK LOSS
Loss of aggregate, also known as rock loss, or shelling, ranks high as one of
the major causes of flushing and bleeding for pavements with seal coats and
surface treatments. Rock loss problems frequently occur when a seal coat is
placed outside the established seal coat season, either late or early.
Rock loss problems tend to happen when abnormally cool or cold temperatures (or rain/snow) occur during or after the placement of a seal coat before
the aggregate/asphalt bond has had a chance to fully develop. Rock loss
problems can also occur when:
•
•
•
•
The binder is too stiff for a particular weather condition.
The asphalt rate is too light.
Too much time elapses between the asphalt shot and rock placement.
Water is trapped in an existing pavement and then sealed. The water
will turn into a vapor and rise when heated resulting in possible rock loss
problems or delamination of the seal coat
TOO MUCH ROCK
The wrong aggregate rate — typically too much aggregate — can cause bleeding and flushing. Where the rock rate is too high, the rock is too crowded
resulting in a solid mat/rocks stacked on top of each other. Here the rock will
crush and create more fines further displacing the asphalt.
The aggregate must have an adequate quantity of voids to accommodate
volume changes of the asphalt with changes in temperature, and at the same
time keep the vehicle tires out of the asphalt. If the rocks are too close together, they cannot “roll over” and get the proper embedment in the asphalt.
Additionally, if the cover stone is too dense (insufficient voids in the mat), the
asphalt will have a tendency to rise above the level of the rock when temperatures rise and the asphalt starts to expand.
Seal coat aggregate should be placed so that rock particles do not touch each
other but are close enough so that a rock will not fit in the space between the
rocks. District personnel like to see 15-20% voids between the rocks. The
freshly placed mat typically looks “light” until the rocks go to their final position. This normally takes a few days under traffic before this occurs.
DIRTY AND/OR SOFT ROCK
Use of dirty seal coat aggregate leads to rock loss. Dirty aggregate causes
loss of adhesion/bond between aggregate and asphalt binder.
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FLUSHED PAVEMENTS
Softer aggregates tend to abrade and wear down (polish out flat) more quickly
under traffic. This can lead to a loss of traction (skid resistance) as well as a
flushed condition and possible subsequent bleeding. This is especially true
when the rock rate is too high. When the seal coat aggregate crushes, this
creates more fines, and the excess fines displace the asphalt (push the oil
up) resulting in less void depth to keep tires out of the asphalt.
Whereas Grade 3 rock is generally more forgiving than the smaller Grade 4
rock with regard to asphalt rates, lightweight rock (usually Grade 3) is very
susceptible to abrasion and wearing down under traffic, especially for high
traffic conditions. Harder rock is recommended for higher volume roads.
MODIFIED AGGREGATE GRADES
Modified aggregate grades (single size rock) have fewer problems with bleeding and flushing. These grades are more predictable, can accommodate a
higher and more consistent asphalt rate, and are more forgiving. In contrast,
normal gradations have more fines, are more variable, and require a lower
asphalt rate. However, aggregate suppliers are reluctant in some instances
to produce and supply modified gradations.
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BINDER ISSUES
GENERAL ISSUES
General binder-related issues associated with flushing and bleeding include
the quality of binder, use of inappropriate (too high or too low) binder application rates, and poor construction practices. Binder properties influence flushing and bleeding of seal coats in two ways. One is when highly temperaturesusceptible asphalt binder is used in the layer below the seal coat. This
binder can soften at very high pavement service temperatures resulting in the
aggregates of the seal coat pressing into the underlying pavement layer. This
pushes the asphalt binder upwards causing flushing and bleeding.
A second issue is incompatibility between the seal coat aggregate and the
binder, which can cause aggregate loss resulting in a bleeding pavement.
This incompatibility may be caused either by problems inherent in the binder,
or by binder-related issues that are accentuated by inappropriate construction practices. Examples of such practices include long delays between the
binder spray and the aggregate spread in the case of emulsion binders, low
binder temperature at the time of aggregate spread for hot asphalt binders,
and inappropriate binder spray temperature for all types of binders.
Another major binder-related issue that causes bleeding is inappropriate curing time used for cutback asphalt binders. When cutback asphalts are used
either in a prime coat or in the first course of a multiple course surface treatment, short curing time for cutback asphalt will leave volatile oils underneath
the seal coat or surface treatment that can significantly soften the surface
treatment binder.
BINDER SELECTION
The appropriate asphalt must be used in the appropriate season as defined
by the project plans, specifications and experience. Softer binders used in
relatively colder climates in the state have been shown to cause more bleeding problems during hot summers in those areas. The use of any asphalt
binder outside the recommended temperature conditions (or asphalt season)
can lead to problems with bleeding, flushing and/or rock loss. This would be
especially true where a cool weather binder is used late in the winter followed
by a period of very warm or hot weather before the binder has a chance to
fully cure and adhere to the rock. Asphalt binders also should be selected
based on the traffic conditions anticipated for the roadway.
HISTORICALLY PROBLEMATIC BINDERS
Some asphalt binders used in the past have since been discontinued or discouraged in certain applications due to problems with flushing and bleeding.
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FLUSHED PAVEMENTS
Examples of the older, less-frequently used binders include unmodified AC-5
and AC-10. When unmodified, softer binders are contained in underlying
layers of pavement or seal coats, they have a tendency to work their way up
through the newer layers and continue to create problems with flushing and
bleeding.
BINDER APPLICATION RATE
Asphalt and aggregate shot rates need to be appropriate for the selected type
and grade of binder and rock. If the asphalt rate is too light it can lead to
rock loss (shelling) and ultimately, flushing. If the asphalt rate is too heavy, it
can lead to bleeding and/or flushing.
Special care must be taken in the material rate design phase and most importantly, during construction, to properly adjust the shot rate for pavements
where previous flushing or bleeding problems have occurred, in wheel paths,
or in other areas rich with asphalt.
Roads that have been shot with too much asphalt can become long-term
maintenance problems because the asphalt keeps softening when subsequent layers are added. Such roadway surfaces eventually have to be replaced.
BINDER CURING
Seal coat and surface treatment asphalts normally require several months of
warm/hot weather before the asphalt gets hard (cured), penetrates well into
the aggregates, and becomes less susceptible to material property changes
with large increases in temperature.
BINDER QUALITY
Poor quality and/or inconsistent binder quality can lead to problems associated with flushing, bleeding and rock loss. This may be due in part to the
quality and variability of the crude (base asphalt) and to binder production
processes adopted by the manufacturer.
Certain districts have had problems with “out of spec” binders received for
field projects. TxDOT’s binder quality assurance program is being updated to
address this issue.
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TRAFFIC ISSUES
TRAFFIC VOLUME (ADT)
Traffic volume and type play a major part in the development of bleeding and
flushing problems. Higher traffic volumes will cause flushing and bleeding to
manifest sooner than for roadways with lower ADT. This is due in part to the
rolling action of the traffic which pushes the rock down into the asphalt or
underlying pavement.
TRAFFIC TYPE (HEAVY TRUCKS)
The amount and percentage of truck traffic (heavy loads) can accelerate aggregate abrasion as well as the process of embedding rock into the underlying asphalt or existing pavement layers. High volumes of truck traffic and
over-weight loads over extended periods of time can also create rutting in the
wheel paths.
Heavy truck traffic, heavy loads, and overweight loads are especially problematic on steep grades or along sharp (tight) curves where frequent braking is
required. These loads are typically associated with certain industries, for
example, farming/agriculture, logging, oilfield, and the like. Because of industry exemptions these trucks are often overweight, sometimes as much as
50 percent. This is particularly damaging to the rural FM roads where such
traffic is most prevalent.
TRAFFIC MOVEMENTS (STOP & GO, TURNING, ETC.)
Starting, stopping (braking), turning, and slow-moving traffic tends to aggravate flushing and bleeding problems. Heavy loads associated with truck traffic make bleeding and flushing worse because such traffic has a tendency to
shove and move the rock. This is especially the case at turns/horizontal
curves.
Braking tends to tip the seal coat rock up, exposing more asphalt binder on
the pavement surface. The rock is more prone to being abraded in this situation.
Turning causes the aggregate to mound up and shove. The reason is that,
unlike cars and most other vehicles, the rear tandem axles of trucks do not
have differentials that allow the rear wheels to turn at different rates as the
vehicle makes a turn. This twisting action of tandem axle tires as they essentially slide across the pavement surface is one of the main causes of scrubbing at intersections and curves. The result is a “washboard” effect on the
pavement surface.
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FLUSHED PAVEMENTS
Slow moving or stopped traffic contributes to many of the problems associated with bleeding pavements, including the tendency to “roll the rock” —
especially for freshly constructed seal coats or surface treatments. This
“rolling up” or “picking up” of the seal coat or surface treatment from the
base or underlying substrate is especially damaging. Slow-moving or stopped
traffic (with hot tires) directed onto fresh seals as part of the traffic control
plan makes an already-difficult problem worse.
INTERSECTIONS
Intersections are particularly problematic relative to slow-moving traffic, starting, stopping (braking), and turning movements. Oil leaks, diesel leaks, and
other solvents tend to be deposited in these areas. Catalytic converters from
slow-moving or stopped traffic produce excessive heat, increasing pavement
temperatures. Turning movements cause the seal coat to be “shoved sideways” resulting in a very irregular (rough) pavement surface. Thus, flushing
and bleeding problems are more prevalent at these areas.
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ENVIRONMENTAL ISSUES
HIGH TEMPERATURE
Temperature and humidity have a profound effect on flushing and bleeding.
High temperatures — nearing 100°F or higher — coupled with elevated humidity can turn flushed pavements into bleeding pavements. This is particularly the case where these high temperatures persist for several consecutive
days, as is common during the summer in most parts of Texas. The severity
of bleeding conditions is made worse in combination with traffic and other
factors described previously.
While no definitive temperature threshold exists to define bleeding, our interviews suggest that ambient temperatures above the mid-to-upper 90°s can
turn flushed pavements into bleeding pavements. Maintenance personnel
report that bleeding can occur at air temperatures of 95°F and above. Data
from an infrared surface temperature gun indicated a pavement surface temperature of 136°F on a bleeding seal coat. Pavement temperature in the
wheel paths can be as high as 150°F with an air temperature of 100°F.
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FLUSHED PAVEMENTS
HUMIDITY
Temperature and humidity conditions for bleeding become more critical if
there is no wind to cool the pavement. The “heat index” is the key — i.e., the
combination of temperature, humidity and wind. High humidity will increase
the severity of bleeding at a given temperature.
Humidity essentially reduces the rate at which a pavement can be cooled.
High humidity also delays the break and set of a seal coat emulsion, and this
can increase the tendency for rock to roll under traffic; traffic has to stay off
the fresh seal for a longer period of time.
High humidity results in slower curing of asphalt (initial setup), especially with
emulsions. This tends to keep asphalt alive longer.
It should be noted that the TxDOT seal coat manual states that “no asphalt
should be shot above a relative humidity of 50%.” While this would be possible in West Texas, this situation simply will not exist for other parts of the
state.
CHANGING TEMPERATURES
Changing weather conditions can present special problems for seal coats and
surface treatments. One example is a series of high temperature days followed by a “cold snap” or cooling rains.
Such unstable weather patterns typically coincide with transition seasons at
the beginning and end of the seal coat season — in the spring and fall of the
year. Inspection personnel have to monitor temperatures closely during construction and shut down seal coat operations completely (or earlier in the day)
if cold weather is expected.
Cool weather/rains during or a few days after seal coat construction can create problems with rock loss which in turn can lead to problems with flushing
or bleeding. Emulsions are particularly susceptible to this situation.
LOW TEMPERATURES
As noted above, cool/cold weather or rain/snow during or shortly after the
placement of a new seal coat can create rock loss problems. This in turn can
lead to problems with flushing and/or bleeding.
The tendency for rock loss is especially severe where a hard freeze is experienced for several days in areas where bleeding has occurred before. The
result is a loss of rock which exposes more asphalt at the pavement surface.
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CONSTRUCTION ISSUES
EXISTING PAVEMENT CONDITIONS
Many of the older farm roads have received numerous layers of asphalt in the
form of multiple seal coats, patches, etc., as well as many different types of
asphalt. Asphalt in subsequent seal coats has a tendency to flow into, build
up or accumulate in rutted wheel paths, increasing the likelihood of problems
associated with flushing and/or bleeding. Special care must be taken on
existing pavements where previous flushing or bleeding problems have occurred, where rutting is present, in patched areas, or in areas rich or deficient
with asphalt.
SEAL COAT PREPARATION
Existing pavement conditions must be carefully considered in advance of seal
coating operations relative to potential bleeding and flushing. Preparation
and repairs on an existing pavement must have adequate time to cure before
the seal coat is applied. This includes level-up operations, spot repairs and
crack sealing. The recommended minimum is 6 months curing time.
Asphalt selection is very important for strip seals and spot seals in advance of
seal coat construction. Certain asphalts such as RC-250 (and others) are
more prone to being “livened up” when hot seal coat asphalt (350°F) is
sprayed on top of them. Asphalt rates must be adjusted to accommodate
each change in condition — dry, hungry or rich areas — otherwise problems
with bleeding, flushing and/or rock loss could occur.
Patching should be done no later than the fall preceding the summer seal
coat season. Fresh hot mix or premix patches have a high demand for seal
coat asphalt. If patches are not adequately cured or seal coat asphalt rates
are not adjusted, the patches can wick the seal coat asphalt resulting in rock
loss which in turn can lead to bleeding and/or flushing. Patches need adequate time to cure and to allow the volatiles to escape before seal coating (1
year preferred, 6 months minimum). If a new seal coat — with hot oil — is
shot over a green patch, traffic can pull up both the seal coat and fresh patch.
Crack sealing should be done at least one year in advance of seal coating.
The crack sealant material is prone to reflect or bleed through a seal coat if it
is not allowed adequate time to cure.
RUTTING IN WHEEL PATHS
Asphalt in subsequent seal coats will tend to flow into, build up or accumulate
in rutted wheel paths, increasing the probability of problems associated with
flushing and/or bleeding. If the wheel paths are already flushed, this should
Page 24
FLUSHED PAVEMENTS
be taken into account when selecting asphalt rates during seal coat design.
During construction, asphalt rates can be adjusted in the wheel paths
through the use of variable rate nozzles or double spray bars. Significant rutting should be corrected before subsequent seal coats are applied.
NEW SEAL COATS
New seal coats are particularly susceptible to bleeding problems immediately
and soon after placement, especially when shot in very hot weather conditions. Construction equipment on the fresh mat can “roll the rock” and “roll
up” the new seal coat if tires are allowed to come into contact with the liquid
asphalt. The traveling public can intensify the problems when the fresh mat
is opened to traffic before the asphalt/aggregate bond has fully developed.
FOG SEALS
Fog seals are used mitigate rock loss problems, and some districts include
provisions in the construction plans for this application. Some fog seals contain a rejuvenator which will liven the asphalt and allow the rock to stick.
However, if the fog seal is overdone, it can create problems with flushing and
bleeding.
POOR CONSTRUCTION PRACTICES
Construction practices which can lead to bleeding and flushed pavements
include, but are not limited to:
•
Sloppy seal coat work – bad joints, overlapping applications of asphalt
and rock, inconsistent asphalt rates.
•
Shoot seal coat out of season.
•
Premature release of seal coat to traffic.
•
Aggregate not spread on the asphalt as soon as possible.
•
Poor rolling equipment and methods; construction equipment (spreader
box and rollers) getting on the fresh seal too quickly.
•
Crack sealants - over application, inadequate curing time and improper
selection of materials.
•
Use of asphalt distributor wands at intersections and radii – rates cannot
be controlled.
Page 25
PREVENTIVE
MAINTENANCE
A PREVENTIVE MAINTENANCE APPROACH
As in medicine, so in maintenance: “An ounce of prevention is worth a pound
of cure.” There is no better advice for dealing with bleeding and flushed
pavements than to avoid the problem from the outset during seal coat/
surface treatment design and construction.
Granted, the treatment of bleeding and flushed pavements is considered
corrective as opposed to preventive maintenance. However, the fact remains
that a preventive maintenance mindset is by far the most cost-effective maintenance solution discussed herein. “You show me a maintenance leader who
approaches seal coat/surface treatment design and construction from the
perspective of how to achieve high quality and avoid bleeding and flushing,
and I’ll show you a maintenance crew that is relatively unbothered by the
bleeding/flushing problem.”
Three topics emerge as being key to a preventive maintenance mindset focused on reducing bleeding and flushing in seal coats and surface treatments. These include avoidance of known pitfalls and problem areas
(potentially avoidable causes), implementation of a centralized seal coat
management program, and judicious selection of seal coat and surface treatment binders.
POTENTIALLY AVOIDABLE CAUSES
The discussion of causes identified aggregate issues, binder issues, traffic
issues, environmental issues, and construction issues as factors that result in
bleeding and flushing. These can be viewed as falling along a spectrum relative to the extent to which they can be directly addressed in seal coat/surface
treatment design and construction.
On one end of the spectrum are the causes which are simply part and parcel
of the seal coat/surface treatment design and construction process and
which cannot be changed. For example, traffic “is what it is.” Similarly, climate and environmental conditions cannot be manipulated by maintenance
forces. In such cases, the best that can be done is to be aware of the conditions that exist and the potential pitfalls associated with use of seal coats/
surface treatments in these applications.
Page 26
AN OUNCE OF PREVENTION IS WORTH A
POUND OF CURE
The other end of the spectrum consists of purely avoidable causes. These
includes things like poor construction practices and procedures, improper
selection of aggregate and binder, and the like.
In between these two extremes are certain situations that seem to regularly
accompany bleeding and flushing problems. These include construction of
seal coats/surface treatments during transition months at the beginning and
end of the seal coat season, placing seal coats in excessively hot weather,
and not providing sufficient curing time before allowing the traveling public on
the new roadway surface.
CENTRALIZED SEAL COAT PROGRAM MANAGEMENT
Districts that utilize a centralized seal coat management program are having
consistent success with their seal coats/surface treatments and fewer instances of problems such as bleeding and flushing. The centralized approach
is an effective model for addressing seal coat design, construction and maintenance.
BINDER SELECTION
Binder selection (both the type and application rate) is key to successful seal
coat/surface treatment performance. The appropriate asphalt must be used
in the appropriate season. The use of any asphalt binder outside the recommended temperature conditions (or asphalt season) for that asphalt can lead
to problems with bleeding, flushing and/or rock loss.
Page 27
POTENTIALLY AVOIDABLE CAUSES
PURELY AVOIDABLE CAUSES
Certain causes of bleeding and flushing in seal coats and surface treatments
are purely avoidable, and they should be avoided. Purely avoidable causes
include things like poor construction practices and procedures, improper selection of aggregate and binder, and the like.
SEAL COAT/SURFACE TREATMENT SEASON
Many problems with seal coats and surface treatments have to do with the
time of year that the seals are applied. Transition months for seal coats can
create many problems. The reason is that early and late season seals are
more susceptible to being applied during cool weather and/or a late cold
spell. Similar problems occur when a cooling rain comes through after the
seal is placed.
Some districts have learned to customize their own seal coat/surface treatment season and have eliminated many of their problems. The seals need to
have adequate time to cure and develop the full asphalt/aggregate bond
before cool/cold weather. This will minimize rock loss and all of the subsequent problems associated with it, including flushing and bleeding.
Surface treatment projects (rehabilitation projects) are the prime offenders
since many of these projects go year round. Construction projects seem to be
where many bleeding/flushing problems start.
Some districts have learned to avoid applying any asphalt in the off-season.
One of the benefits of this approach is that maintenance forces do not have
to use cool weather asphalts. Ultimately, fewer problems occur when seal
coats are placed in the asphalt or seal coat season.
HOT WEATHER SEAL COATS
Cool weather is not the only challenge for seal coat/surface treatment construction. Many seal coat problems begin when contractors shoot seals during protracted periods of extremely hot weather (upper 90s and 100s) and
prematurely open the road to traffic. In such cases, the asphalt/aggregate
bond does not have a chance to fully develop.
This situation can result in rolling rock, severe bleeding, and “picking up” of
the seal coat or surface treatment by the traveling public. Heavy construction
traffic can also severely damage the seal coat mat before the public ever gets
on it. This is one reason why contractors should be encouraged to keep water
trucks full of lime water available on the jobsite. The point is that adequate
Page 28
POUND OF CURE
curing time is needed for development of complete adhesion between the
rock and the asphalt, and this is especially critical in hot weather under conditions of high traffic volume and heavy truck traffic.
MULTIPLE LAYERS OF SEAL COATS
System roadways typically have multiple seal coats, and a common scenario
is when maintenance forces attempt to address bleeding/flushing on these
roads with yet another seal coat. The new seal coat may stop the bleeding/
flushing for a period of time; however, this is usually only a temporary fix.
The bleeding/flushing problem often continues and may increase in severity
due to adding more asphalt. Rather than repeatedly applying more binder,
consideration should be given to removing excess asphalt (flushing) during
the seal coat preparation process in advance of seal coating operations.
INADEQUATE CURING TIME
One cause of bleeding is when the traveling public is allowed on the fresh
seal too quickly, before the asphalt/aggregate bond has been given adequate
time to fully develop. This is most critical in adverse weather conditions and
on roadways with high traffic volumes and heavy truck traffic.
A different kind of “curing time” poses another problem. This has to do with
construction projects that use cool weather asphalts including emulsions and
cutbacks. Many of the major offenders (bleeders) fit into this category, including emulsions such as CRS-1P and cutbacks such as RC-250, MC-2400,
MC-3000, etc. The common thread is that volatiles, consisting of lighter fraction oils, kerosene, diesel, naphtha and even gasoline, are not allowed sufficient time to cure out before the seal is covered up with another course.
Emulsions and cutback materials are typically used as a prime coat on base
or for the first course of a multiple course surface treatment. Prime coats
require considerable periods of warm or hot weather for the light fraction
cutbacks to cure out (volatilize).
Even though standard specifications require a certain number of minimum
curing days for each type of material, the specifications do not define what a
“curing day” is, and temperature and wind play a big factor. Some of the most
severe and difficult bleeding problems occur as a result of volatiles being
trapped inside (or beneath) a multiple course surface treatment or seal coat.
Page 29
CENTRALIZED SEAL COAT PROGRAM
MANAGEMENT
DESCRIPTION
Successful seal coat construction occurs where districts have a centralized
seal coat program together with an experienced and committed seal coat
team that is dedicated to quality.
Leadership is provided by the Seal Coat Program Manager. Typically this is a
very experienced individual who is responsible for the entire district, and seal
coats are this person’s full-time priority. Other team members include seal
coat professionals from district headquarters, area offices and maintenance
sections who are responsible for making the program work year in and year
out. Districts that employ this centralized model are enjoying consistent success with seal coat construction and maintenance.
Quality seal coat construction happens as a result of a team effort between
the design, maintenance and seal coat construction groups. The process
requires good communication and everyone working together. The seal coat
team is responsible for every aspect of the seal coat program, from planning
to construction inspection.
SEAL COAT PLANNING
The seal coat planning process starts with a map of the roads to be sealed,
and this map would be developed 2 to 3 years before actual seal coat construction. Many individuals provide input on project selection and priorities
including the Seal Coat Program Manager, Area Engineers, Maintenance Section Supervisors, the Director of Maintenance, and the Director of Construction.
The Seal Coat Program Manager will typically visit the roads at different times
of the year before seal coating. Traffic patterns tend to change throughout
the year and these visits help the seal coat team get a better overall picture
of the traffic on the roadways. The visits also help the team evaluate truck
traffic types and levels since published ADT’s do not normally reflect actual
truck percentages.
Planning is critical. A multitude of factors are considered including existing
pavement condition, skid resistance values, accidents, time since the last
seal coat, and of course the budget. The number of years since the last seal
coat is not always the key. The pavement condition (dry, rich, oxidized, etc.)
and distresses such as rutting, bleeding, flushing, rock loss and cracking play
a major role in the planning process.
Page 30
POUND OF CURE
MAINTENANCE PREPARATION
Once the roads are chosen to be seal coated, several tasks proceed concurrently including maintenance preparation and seal coat design. Maintenance
plays the major role in the seal coat preparation process. Maintenance crews
are responsible for performing the necessary repairs to their roadways in
advance of seal coat operations.
Seal coat preparation includes tasks such as crack sealing, repair of base
failures, rutting repairs, edge repairs and level-up patching to name a few.
Ideally, all patching and crack seals should be completed at least one year in
advance of seal coat construction to allow any volatiles to cure and the
patches to stiffen up. Good compaction and curing of the patches is critical
so the overlying seal coat will not embed into a soft patch creating a potential
flushing and/or bleeding problem.
Significant rutting must be repaired before a road is sealed. The establishment of a good cross slope is very important in minimizing the problems associated with rutting. If the rutting is not corrected, there is a tendency for asphalt to flow towards and accumulate in the bottom of the ruts when a new
seal coat is placed. This will often lead to premature flushing and bleeding in
the wheel paths.
DESIGN
Another important part of the process is design. The Seal Coat Program Manager is typically is responsible for seal coat design which includes material
selection (asphalt and rock), development of asphalt and rock base rates, use
of variable rate nozzles, and preparation of plans.
Material selection is mostly based on experience and on what has worked
well in the past. Design procedures such as the modified Kearby method can
be used, and these methods are typically adjusted to account for local experience. Once the materials are selected and the base rates established, seal
coat plans are developed, finalized and put out to bid.
ASPHALT AND AGGREGATE APPLICATION RATES
Proper asphalt and aggregate rates are critical to the success of every seal
coat or surface treatment. The seal coat team, under the leadership of the
Seal Coat Program Manager, typically calls all of the asphalt and aggregate
rates during the construction process. Asphalt rates are established on the
fly and are marked on the road in advance of the seal coat construction crew.
Page 31
CENTRALIZED SEAL COAT PROGRAM
MANAGEMENT, CONT’D
The rates are based on the actual pavement conditions at the time of construction and can be further tweaked to account for changes in the weather
and contractor performance.
The goal is to apply the correct amount of asphalt to the pavement surface. A
subtle threshold exists between too much asphalt (flushing and bleeding) and
too little asphalt (rock loss). The default position is to stay at the upper limit
of the asphalt rate, the idea being that flushing is preferred to loss of rock.
SEAL COAT INSPECTION
A knowledgeable and experienced inspection staff is critical to the success of
any seal coat program. Inspection crews ideally stay together as a team and
follow the seal coat contractor to each location. Inspectors from each maintenance section supplement the seal coat inspection team whenever the contractor is working in their area. These maintenance section inspectors complement the chief inspectors with their local knowledge in addition to helping
with rock or asphalt inspection.
Page 32
POUND OF CURE
The Seal Coat Program Manager is responsible for the development and training of a core of experienced inspectors. Experience is always the key to success and is the best long term investment in the seal coat program. Full-time
maintenance inspectors have a tendency to take ownership of the seal coat
program as compared to the construction inspector who spends most of his
time on construction projects. This approach results in more consistent quality seal coats with fewer problems.
New inspectors are put on the job under the supervision of the more seasoned inspectors and quickly learn that seal coating is more of an art than a
science. Plan rates must be adjusted to accommodate varying field conditions, and inspectors need the ability to be flexible. There is a very fine line
between success and failure; therefore, it is critical that an experienced inspector be on each job.
A trained inspector needs to be aware of the variations in aggregates and
asphalts and must know when to change rates and why. It normally takes
about 4 years, or seal coat seasons, of training in the field before an inspector is knowledgeable enough to take on more responsibility.
Page 33
BINDER SELECTION
SEASONAL CONSIDERATIONS
The use of binders modified with latex, polymer and crumb rubber has significantly reduced bleeding problems across the state. The superior performance of these modified binders may be attributed to reduced temperature
susceptibility, enhanced high-temperature properties and improved aggregate-binder bonding effectiveness.
Binder selection (both the type and application rate) is key to successful seal
coat/surface treatment performance. Relative to bleeding/flushing, certain
binders are more forgiving than others. The appropriate asphalt must be
used in the proper season.
WARM WEATHER BINDERS – CONTRACT SEALS
Unmodified asphalt cement (AC) binders such as AC-5, AC-10 and AC-15 were
used for seal coats and surface treatments before the advent of the tire rubber (TR) and other polymer modified binders. Bleeding was very common
when the AC binders were used. In contrast, the high-performance modified
binders have dramatically reduced problems with bleeding.
Most districts are currently using tire rubber and polymer-modified binders as
their warm weather binders of choice for seal coats and surface treatments,
especially on higher-volume roads. Preference between the types of binders
varies between individuals.
AC-15P is another binder that is commonly used for warm weather seal coats.
The AC-15P is softer than the high-performance modified binders, and the
softer asphalt will normally hold the rock better and have fewer problems with
rock loss for low-traffic roads. In contrast, the stiffer (harder) asphalts tend to
pull away from the rock when placed in a lower traffic environment. For this
and other reasons, some districts correlate binder choice with traffic according to the following rule of thumb:
•
ADT greater than 15000… use high-performance modified binder
(example: AC-20-5TR and AC-20XP)
•
ADT between 1000 to 15000… use modified hot asphalt binder
(example: AC15P, AC10 with latex, AC5 with latex)
•
ADT less than 1000… use emulsion
(example: HFRS-2P or CRS-2P)
Certain districts use hot rubber asphalt (HRA) which they identify as being
very forgiving with regard to asphalt rate and performance, and also very expensive. HRA is produced with AC-10 base asphalt and about 22 percent
Page 34
POUND OF CURE
crumb rubber and a “sun screen” inhibitor to minimize photo-degradation.
The hot rubber seal requires a higher asphalt application rate than comparable binders and is considered to be very resistant to long term problems with
flushing and bleeding.
WARM WEATHER BINDERS - IN HOUSE SEALS
Maintenance personnel normally use a CRS-2P (emulsion) for their warm
weather “in-house” seals. This binder is normally used when air temperatures are 60°F and rising. Other in-house binders include HFRS-2P and
CRS-2h. Maintenance forces will also use CRS-2 and RC-250 over base.
Most emulsions perform fairly well as long as traffic volume is low (ADT less
than 1000) and the road is not subject to abusive movements – turning, stop
and go, etc. – that occur at intersections. Some flushing and bleeding is reported with higher traffic volumes and heavy truck traffic.
COOL WEATHER BINDERS – CONTRACT SEALS
Cool weather binders are used on construction projects for surface treatments or underseals, or for maintenance outside the traditional seal coat
season. Contractors typically choose from CRS-2P, HFRS-2P, CRS-1P, and
others. The choice is frequently a material availability issue where the contractors use whatever is being produced by the asphalt suppliers at the time.
Maintenance personnel report that emulsions placed in cool weather have
experienced problems with bleeding later on — in warm to hot weather. Numerous complaints, for example, have been lodged about CRS-1P due to
major problems with bleeding. The suggestion is to avoid emulsions during
the winter unless there is no other alternative. Waiting –delaying construction – is a viable alternative and may well be the best approach. Emulsions
are particularly troublesome in high traffic, at intersections, and under heavy
truck traffic.
COOL WEATHER BINDER – IN HOUSE SEALS
Asphalt suppliers typically stop making hot ACs between December and January, so maintenance forces end up having to use emulsions and/or cutbacks
on construction projects during cool weather. Maintenance personnel prefer
to avoid shooting asphalt during cool weather if at all possible.
Some maintenance forces utilize CRS-1P (emulsion) or RC-250 (cutback) for
their cool weather binders. However, each of these binders can create problems with bleeding and flushing. The materials are very sensitive to the volume and type of traffic and have experienced bleeding problems in the past.
Page 35
BLEEDING
CORRECTIVE (REQUIRED) MAINTENANCE
Bleeding asphalt pavement is an immediate maintenance problem that must
be addressed. The conditions that call for treatment of a bleeding asphalt
pavement include:
• The asphalt begins to soften and liquefy on the roadway surface.
• Vehicle tires start tracking liquid asphalt down the highway.
• Periods of hot weather and high humidity are anticipated (one or more
days of 100°F or greater temperatures).
• TxDOT personnel receive complaints from the traveling public about
asphalt on their vehicles.
• The wet asphalt begins to stick to vehicle tires, and in turn, starts to
“pick up” or “roll up” the seal coat or surface treatment wearing course
on the tires.
Maintenance personnel seek to treat a bleeding asphalt pavement before it
starts to pick up to avoid the need for more extensive and costly pavement
repairs.
MAINTENANCE TREATMENT STRATEGY
Maintenance forces employ several methods to treat bleeding asphalt pavements. The basic approaches are (a) to bridge over the liquid asphalt by applying aggregate of various types and gradations, (b) to cool off the pavement
surface by applying water with or without additives, or (c) to remove the
bleeding asphalt and rebuild the pavement seal. The method chosen often
depends upon the available materials, manpower and equipment at the time
of treatment.
The selection of the treatment approach must also consider the severity of
the bleeding problem as well as many other factors including environmental
conditions (temperature and humidity), type of roadway, traffic levels and
types, and specific locations on a roadway (curves, intersections, urban or
rural environments).
Maintenance personnel indicate that many of the treatment methods are
employed during the hottest times of the day when bleeding starts or is already active. The latter condition would be preferable when sand or aggre-
Page 36
SOLUTIONS FOR BLEEDING ASPHALT
PAVEMENTS
gate materials are used as part of the treatment, the reason being that the
asphalt must be liquid, hot, and sticky for the aggregate materials to adhere
to the pavement surface.
SUMMARY OF MAINTENANCE SOLUTIONS
The following table summarizes maintenance solutions for treatment of
bleeding asphalt pavements.
Maintenance
Solutions for
Bleeding Asphalt
Pavement
Solution Type
Bridge
Over
Apply layer of small
size (Grade 5)
aggregate
■
Apply layer of
larger size (Grade
4 or 3) aggregate
■
Apply blotter material (coarse sand/
stone screenings)
■
Sandwich seal
Cool
Down
Remove
Replace
Short
Term
Mid
Term
■
■
■
Apply water
Long
Term
Low
Medium
High
■
■
■
■
■
■
■
■
■
■
Remove bleeding
pavement surface
& replace
■
■
Cost
■
■
Apply lime water
Other methods
Effectiveness
■
■
■
■
■
■
■
■
■
Page 37
APPLY LAYER OF SMALL SIZE
(GRADE 5) AGGREGATE
DESCRIPTION
The application of Grade 5 aggregate is the most commonly-used maintenance solution for the treatment of bleeding asphalt pavements in Texas.
Maintenance forces use this option if there is enough free asphalt on the
pavement surface to “stick” the rock. Construction contractors also use this
method.
APPLICATION
This method is typically used to treat light to moderate bleeding. The key with
any aggregate remedy is to get the rock to stick to the pavement surface, and
the primary objective is to get the tires out of the asphalt. Some maintenance
forces use this treatment in conjunction with lime water.
EFFECTIVENESS
Effectiveness varies with this method. This can be a short-term solution (1 to
3 days) or mid-term solution (several weeks), with success dependent upon a
variety of factors including the severity of bleeding, temperature and humidity, traffic volume and type, pavement construction type and history (materials
and methods), location (urban, rural, intersections, etc.), and others.
Page 38
PAVEMENTS
MATERIALS
TxDOT standard specification Item 302 defines Grade 5 rock as having a
nominal size of 0.19 inch (number 4 sieve). The particle size is smaller than
1/4 inch, but larger than 0.09 inch (number 8 sieve).
While maintenance crews use the aggregate they have, preferences for a
particular gradation or type of rock have been voiced on all sides of the issue.
This has included modified (single size) versus standard gradation, lightweight versus hard rock aggregate, either precoated or uncoated. Local material availability and experience are the best guide in this instance.
PROCEDURE
Maintenance forces plan their treatment approach on a case by case basis.
A one-pass treatment might be adequate for light bleeding; whereas, more
severe bleeding might require a series of applications over several days.
Page 39
APPLY LAYER OF SMALL SIZE
(GRADE 5) AGGREGATE, CONT’D.
An aggregate spreading operation typically uses one dump truck with a tailgate spreader (one driver and one spreader operator), one steel wheeled
roller with operator, and a crash attenuation vehicle with driver. Sometimes
an additional dump truck is used. Traffic should be slowed or stopped
(controlled) during the treatment process.
Treatments are typically scheduled during the hotter times of the day to maximize the chances for the rock to adhere to the asphalt.
The actual method of operation depends on traffic and circumstances. Crews
frequently use a tailgate spreader on a dump truck to spread the rock, typically in the wheel paths only. However, crews may use a sand spreader when
they want to get complete coverage across the entire road. The aggregate
can be applied by hand (with shovels) for smaller areas. The dump truck is
normally backed as the aggregate is spread for short stretches of road, and
driven forward for longer sections.
The Grade 5 rock can be rolled with a small flat-wheel roller to seat the aggregate into the bleeding asphalt and to maximize the probability of rock adhesion. This is especially the case for larger treatment areas with a higher volume of traffic. In instances where a roller is not available, or where its use is
not practicable, crews may use the dump trucks to roll the rock into the pavement surface. In some instances, usually on lower volume roads, crews may
depend on the traveling public to roll the smaller aggregate into place.
HELPFUL TIPS
Maintenance personnel recommend “catching it early” – as soon as the
bleeding starts and the asphalt is hot enough – so the rock will stick. It is
important to not let the bleeding get too far out of hand before applying the
first treatment.
Maintenance personnel have used rotary brooms to sweep excess rock
across a bleeding pavement (lane) in lieu of another application of rock. The
swept rock sticks to the bleeding asphalt and traffic rolls it in. A shadow vehicle is used for this operation.
Some maintenance personnel would rather use two light to moderate treatments with Grade 5 rock as opposed to one very heavy application.
Some have included a bid item in seal coat construction contracts for Grade
5 rock (material only). This ensures that the material is available in the event
it is needed to treat bleeding during construction.
Page 40
PAVEMENTS
CONCERNS
Given that a seal coat needs adequate voids between the rocks to accommodate asphalt volume changes, the use of finer aggregate materials such as
Grade 5 rock, and more particularly screenings and/or sand, can make a
bleeding situation worse. This occurs where the smaller rock continues to
displace the asphalt to a higher level, above the seal coat rock. It is on this
basis that some prefer to use Grade 5 modified (single size) rock because it
has significantly fewer fines than the standard gradation.
One of the main concerns in the maintenance community relative to the use
of aggregates is the potential for windshield damage if the rock does not
stick. The concern escalates as the size of the aggregate particles increases
– Grade 3 being the largest size, Grade 5 being the smallest.
Page 41
APPLY LAYER OF LARGER SIZE
(GRADE 4 OR 3) AGGREGATE
DESCRIPTION
The application of larger-sized aggregate – Grade 4 or Grade 3 rock – is another solution for bleeding asphalt pavements, the objective being to get the
tires out of the asphalt. Ideally, maintenance personnel will use the largest
size rock practicable to remediate bleeding, the key question being whether
enough free asphalt exists for the larger aggregate particles to adhere to the
bleeding pavement surface.
APPLICATION
Larger aggregates (Grade 3 and Grade 4) are used when bleeding is severe.
This solution is typically employed by construction contractors for treatment
of bleeding that occurs during or shortly after seal coat construction. Here,
the contractor applies the same rock that was used for the original seal coat,
these original aggregate materials being still available (stockpiled) on site.
The use of larger-sized aggregate materials by maintenance crews is less
common, simply due to the challenge of getting the rock to stick. However,
when conditions are appropriate, maintenance crews do use Grade 4 aggregate for the more severe bleeding problems.
Page 42
PAVEMENTS
EFFECTIVENESS
When properly done, this treatment method is considered a long term solution. Effectiveness depends on many factors including the quantity of free
asphalt, binder type, temperature, traffic volume and type, underlying pavement condition, and others.
MATERIALS
nominal size of 1/4 inch. The particle size is generally smaller than 3/8 inch,
but larger than 0.19 inch (number 4 sieve).
nominal size of 3/8 inch. The particle size is generally smaller than 1/2 inch,
but larger than 1/4 inch.
As noted above, when construction contractors use larger-sized aggregate, it
is typically the same as the original seal coat rock. Maintenance crews, however, tend to use only Grade 4 or Grade 4 modified (single size) rock for this
solution. Maintenance forces typically do not use Grade 3.
PROCEDURE
Placement and rolling of the Grade 4 or Grade 3 rock is accomplished in
much the same manner as for the Grade 5 rock (see previous description).
Timing of this treatment is essential to success. The temperature must be
hot enough for the rock to stick when it is applied – 1:00PM to 7:00PM would
be appropriate in the summer (the hot time of the day).
The quantity of asphalt must be sufficient to stick the rock to the pavement
surface. Although not commonly done, a light layer of hot asphalt can be shot
over the affected (bleeding) area if sufficient asphalt is not available. This
shot of relatively hot asphalt will heat up the underlying asphalt layer enhancing the ability of the rock to stick to the pavement surface.
The aggregate is usually spread with a chip spreader, tailgate spreader, or a
sand spreader. A light pneumatic tired roller is preferred to seat the largersized rock in the asphalt; however, tandem truck tires may be used if a pneumatic roller is not available. A steel wheeled roller should not be used if it will
crush the rock.
Page 43
APPLY LAYER OF LARGER SIZE
(GRADE 4 OR 3) AGGREGATE, CONT’D.
HELPFUL TIPS
TxDOT will occasionally purchase back excess seal coat rock (Grade 3 and
Grade 4) from a contractor after construction and leave it stockpiled by the
road in the event maintenance forces need to re-rock an area or treat a
bleeding asphalt pavement.
Some maintenance personnel prefer lightweight aggregate because there are
fewer problems with windshield breakage. This rock is more uniform in size
and thus is more effective relative to separating tires from the binder. Also,
lightweight aggregate is more porous such that it may absorb excess asphalt.
CONCERNS
The key to success for this treatment option is getting the larger aggregate to
stick to the bleeding asphalt pavement surface. Even when the bleeding
problem is severe, the amount of hot liquid asphalt on the roadway may be
insufficient. This is because the pavement temperature for bleeding asphalt
Page 44
PAVEMENTS
is typically around 140°F to 165°F whereas the original application temperatures for hot asphalts range from 300°F to 350°F.
The main concern in the maintenance community relative to the use of largersized aggregates is the potential for windshield damage due to flying aggregate. Windshield damage and complaints/damage claims from the traveling
public are more of an issue with the larger-sized aggregate.
Page 45
APPLY BLOTTER MATERIAL TO BLOT UP
EXCESS ASPHALT (COARSE SAND/STONE
SCREENINGS)
DESCRIPTION
Aggregate with finer particle sizes (smaller than Grade 5) – for example, sand,
ice chatt, bottom ash, crushed stone screenings, and the like – can be used
as blotter materials to soak up excess asphalt from the bleeding pavement
surface. As with any aggregate solution, the objective is to get the tires out of
the asphalt.
APPLICATION
Maintenance forces sometimes do not have access to their material of choice
for treatment of a bleeding asphalt pavement and are constrained to use
whatever is on hand at the time. Sand and similar finer-grained blotter materials frequently are viewed as being in this category – they are used to provide
temporary complaint relief when nothing else is available.
Under the best of circumstances blotter materials are employed to treat light
bleeding – that is, between a flushed and bleeding pavement condition. As
Page 46
PAVEMENTS
bleeding becomes more severe, the finer-grained materials become increasingly problematic.
EFFECTIVENESS
This treatment method is typically considered to be a short term, temporary
solution requiring multiple applications. Maintenance crews report that they
get about 1 to 3 days of relief from bleeding when using this method. However, the diversity of materials represented by this category – ranging from
manufactured ice chat to blow sand swept from the bar ditch – is such that
effectiveness varies.
Some maintenance crews prefer finer-grained blotter materials over Grade 5
rock because of availability, lower cost, and less concern with windshield
damage. Others consider the finer-grained materials to be a stop-gap treatment, used as “a last resort” and “better than nothing.” Ultimately, effectiveness must be evaluated on a case by case basis.
MATERIALS
The sand and other fine-grained blotter materials have gradations smaller
than Grade 5 aggregate, but typically do not have a defined material specification. The more commonly-used blotter materials include:
•
Ice chatt… used for sanding roads and bridges during ice and snow
events (also known as “ice rock”).
•
Uncoated crushed rock or stone screenings… a byproduct from the standard aggregate grade production process, yielding a material which is
smaller than Grade 5 aggregate.
•
Bottom ash… fine grained sand with some medium and coarse grained
particles created as a by-product of power generation from coal-fired
electric generation plants.
•
Field sand, blow sand, creek sand… a fine to medium-grained, sandy,
naturally-occurring material obtained from along the shoulder of a road
or from a creek bank.
•
Blended materials… bottom ash or sand blended with Grade 5 rock.
Page 47
APPLY BLOTTER MATERIAL TO BLOT UP
EXCESS ASPHALT (COARSE SAND/STONE
SCREENINGS), CONT’D.
Performance of a particular material will be a function of its gradation and the
nature of the aggregate. Some of the finer-grained materials, especially the
ice chatt and the crushed stone screenings, are considered to be functionally
equivalent to Grade 5 rock. That is, like Grade 5 rock, these materials serve
to get the tires out of the asphalt rather than to blot up excess asphalt.
PROCEDURE
Application and compaction of sand and other finer-grained blotter materials
is accomplished in much the same manner as for the Grade 5 rock (see previous description).
Crews typically use a tailgate spreader on a dump truck and spread a thin
layer of blotter sand in the wheel paths only. Alternatively, the blotter materials may be broadcast full-width over an entire lane with sand spreaders similar to those used to spread deicing rock during the winter. A pickup truck bed
may be used to transport/spread screenings on smaller areas (with shovels).
Maintenance personnel indicated that they have also used a rotary broom to
sweep relatively clean sand and dirt (without trash) from the roadside onto a
bleeding pavement surface when nothing else was available.
The dump truck is normally backed as the blotter sand is spread, and crews
typically depend on the traveling public to roll these materials into place.
However, the rock can also be seated in the bleeding asphalt by a pneumatic
tired roller, the claim being that rolling yields a slightly longer-lasting solution.
Timing of this treatment is essential. These materials should be spread in the
heat of the day when bleeding is active and there is liquid asphalt on the
pavement surface.
HELPFUL TIPS
This option is economically viable when maintenance resources run low of
preferred aggregate materials toward the end of the year.
Some claim that the fine-grained blotter materials work well for minor bleeding. The sand and dust “kill” the oil and the wind/traffic blow excess material
off the road.
Blotter sand/screenings must be completely dry. Otherwise, the material has
a tendency to “patty up” or clump together with the liquid asphalt.
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PAVEMENTS
CONCERNS
When evaluating aggregate for treatment of bleeding asphalt pavements, one
way to think about particle size is like the story of Goldilocks and the Three
Bears – the rock can be too large, too small, or just right. Most maintenance
personnel would say Grade 5 rock is the “just right” option, and if so, sand
and the other finer-grained blotter materials are in the “too small” category.
A seal coat requires adequate voids between the rocks to accommodate asphalt volume changes, and for this reason the application of finer aggregate
materials such as blotter sand can make a bleeding situation worse. This
occurs when the smaller particles displace the asphalt to a higher level,
above the seal coat rock. Although the finer materials have a lot of surface
area to absorb the asphalt, they also have a tendency to make “mud” and to
“slick up” the road surface, creating skid problems.
Because traffic will push the fines down into the asphalt and raise the asphalt
layer, some believe that blotter material should be limited to use as an
“emergency treatment” only. One individual commented that blotter material
can turn a 2 or 3 day bleeding problem into a summer-long bleeding problem.
Page 49
SANDWICH SEAL
DESCRIPTION
The sandwich seal is a two-course surface treatment where aggregate is
spread on an existing binder-rich surface before the application of a singlecourse surface treatment. It is very much like a two-course treatment except
the first application of binder is omitted.
Coarse stone is placed directly on the road with no binder underneath it. A
single application of binder is then sprayed on the uniformly-spread coarse
stone, followed by a second application of aggregate using smaller stone. The
new seal is then rolled.
This method is similar to and sometimes confused with an upside-down or
inverted surface treatment. However, for the inverted seal the first layer of
aggregate is the finer material and the second layer is coarse.
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PAVEMENTS
APPLICATION
While the sandwich seal directly addresses the mechanisms associated with
moderate to severe bleeding (displacement of asphalt/loss of voids), this
method has seen limited use within TxDOT. The most common application
has been as a type of strip seal to correct moderate to severe bleeding and
minor rutting in the wheel paths.
The sandwich seal is suited for treatment of chronic bleeding pavements
where the maintenance section does not have the money to mill and inlay
with hot mix.
The sandwich seal is not considered a maintenance seal in that it is typically
not done by maintenance personnel. The sandwich seal is usually applied by
a contractor or by the district Special Jobs Crew.
EFFECTIVENESS
This treatment method seals the roadway surface and improves skid resistance. It is considered to be a long term solution to bleeding.
MATERIALS
Asphalt binder, coarse aggregate, and fine aggregate for the sandwich seal
are typical materials for seal coats and are selected on a case-by-case basis.
Materials should conform to TxDOT Standard Specifications.
PROCEDURE
To construct a sandwich seal, first, a clean coarse aggregate (seal coat rock)
is spread on top of the bleeding asphalt. A layer of asphalt binder is then
sprayed on top of the untrafficked coarse aggregate, and then a second layer
of finer seal coat rock is placed over the asphalt.
The asphalt rate for the sandwich seal is normally cut about a third from the
original seal coat, recognizing that application rates differ widely for different
binders.
The rock rate for each course of the sandwich seal should be around 98-100
sy/cy assuming a Grade 3 seal coat rock is used for the first course. A pneumatic tired roller is typically used to roll the rock.
For best performance, this treatment method must be used when the temperature is at its highest level and the asphalt is hot and sticky.
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SANDWICH SEAL, CONT’D.
HELPFUL TIPS
The sandwich seal is typically applied as a strip seal to correct moderate to
severe bleeding and minor rutting in the wheel paths.
This method yields exceptional friction or skid resistance properties with good
macrotexture in the surface.
Another benefit is that no coarse aggregate is loose and flying, considerably
decreasing the chances of headlight and windshield breakage.
CONCERNS
The sandwich seal is a relatively complicated remediation method for bleeding pavements that involves three separate materials including two sizes of
rock and an asphalt binder. This might be somewhat awkward for a maintenance section crew, the main challenge being equipment availability.
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PAVEMENTS
This method might fit into one of the more expensive remediation categories
that would be better suited for contractor applications in larger areas.
The main challenge is having an adequate quantity of asphalt on the bleeding
pavement surface to stick the first layer of coarse aggregate. To achieve consistency it may be necessary to spray at least a light shot of binder (tack coat)
to hold the coarser rock. Thus the adhesion between the existing bleeding
asphalt pavement and the first layer of coarse rock is the main concern. If
the first layer starts to delaminate or lose its bond with the underlying pavement, flying and/or peeling chunks of sandwich seal may occur.
Page 53
APPLY LIME WATER TO COOL AND CRUST
OVER BLEEDING
DESCRIPTION
The application of hydrated lime mixed with water is one of the most commonly-used maintenance solutions for the treatment of bleeding asphalt
pavements in Texas. Customary practice is to mix lime water using a portable
1000 gallon water tank unit that slides into the back of a dump truck. The
tank unit comes with an agitator pump to keep the lime in suspension, and
the lime water is applied to the bleeding pavement surface using a spray bar.
APPLICATION
This method is typically used to treat light to moderate bleeding on roadways
that do not experience extremely high volumes of traffic. Both construction
contractors and maintenance crews use this method, and they like it because
it is quick and inexpensive and because it is a moving operation. A typical
application is on freshly placed seal coats to control bleeding and to minimize
the chances of the seal coat being picked up.
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PAVEMENTS
EFFECTIVENESS
The application of lime water is generally viewed as a short term solution to
bleeding asphalt pavements – it “buys some time.” Maintenance crews indicate that treatment effectiveness can range from as little as 2 to 4 hours to
as much as 3 to 5 days. Multiple applications of lime water are normally required to achieve the desired result.
It is believed that the lime oxidizes (ages) the hot liquid asphalt, reduces the
stickiness, crusts over the asphalt surface and turns the asphalt green in
some cases – the color change indicates the lime is working. The lime powder chemically reacts with (adheres to) the hot asphalt particles and “kills”
the asphalt, causing it to lose its “livened state”. The lime water treatment
also paints the road white, thereby reducing the pavement temperature due
to the increased reflectivity.
Lime water essentially provides a temporary cooling effect on the pavement
surface. The cooling effect can last for as long as 2 or 3 days but pavement
temperature will begin to climb again if air temperatures hover in the 100°F
range for an extended time. The cooling effects eventually go away and additional treatments are required to maintain the lower pavement temperatures.
Residual lime may also provide a temporary separation between the tires and
the asphalt.
MATERIALS & EQUIPMENT
No standard specification exists for the lime water method, so considerable
variation in exists in treatment practices.
A successful lime water operation requires, at a minimum, Type A hydrated
bag lime (50 pound bags, typical) and a potable water source.
The lime water treatment setup typically consists of one dump truck that contains a portable 1000 gallon water tank. These water tanks can be purchased from several sources and normally come with a 7-foot spray bar and
water pump which is capable of providing continual circulation of the lime
water mixture. Lime and water do not mix very well; therefore, it is very important to have a pump and hose mounted on the portable tank to effectively
circulate and mix the lime water solution. The powdered lime needs to be
circulated in the water tank for at least 5 to 10 minutes to be properly mixed.
Constant circulation is critical.
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OVER BLEEDING, CONT’D.
The lime water is distributed by gravity flow through ½-inch diameter nozzles
which are normally spaced along the spray bar on 2-inch centers. The maintenance crew normally plugs all of the nozzles in the spray bar except for
those in the area of the wheel paths. It is usually not necessary to treat areas
outside the wheel paths since bleeding frequently is limited to that area.
The operation requires a second truck which serves as the crash attenuator
(traffic control vehicle). The crew is normally comprised of two persons, one
driver for each truck. The crash attenuator truck follows the water truck at a
distance that will ensure the pavement has fully dried before traffic is allowed
back onto the treated roadway.
PROCEDURE
Factors such as the air temperature, pavement temperature, humidity, severity of bleeding, tackiness of the asphalt, type of asphalt, type of roadway
(urban, rural, number of lanes), traffic levels (ADT, truck traffic), roadway loca-
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PAVEMENTS
tion (intersection, driveways, straight stretches of road) and time of day all
come into play when defining the details of the lime water treatment application. Each bleeding asphalt pavement problem should be evaluated on a
case by case basis.
It can be beneficial to wait until it gets “real hot” – 100°F and greater, with
pavement temperatures on the order of 130°F-140°F – before treating a
bleeding pavement with lime water. Maintenance forces have recorded pavement temperatures as high as 140°F-150°F in areas where bleeding is present, compared with 115°F outside the area of bleeding.
The rate of application is a function of the water truck speed, among other
things. Maintenance forces typically operate the water truck between speeds
of 25 to 45 mph, with speeds of 35 to 40 mph being customary. This operating speed is necessary to achieve a good splatter (dosage) of the lime water
solution on the affected pavement surface. If the water truck moves slower
than the recommended speed, lime water will start to accumulate in puddles,
resulting in lime water spray on the cars. The lime water will usually “spread
out” across the lane as the operation moves down the highway.
Page 57
The lime water application on the pavement should be dry before allowing
vehicles back on the treated roadway. Lime water treatment is sensitive to
temperature, humidity, and wind (the heat index). In particular, if the humidity is high the lime water solution will not evaporate as fast but instead will
tend to puddle on the road. It is undesirable for the lime water solution to
remain on the pavement for 10 minutes or more. The objective in all situations is to keep the traveling public out of the lime water as it will splash on
their vehicles and result in complaints.
Recipes for lime water concentrations vary with individuals and is more art
than science. An incremental approach to lime water treatment is the recommended practice. This approach starts with a higher lime concentration for
the first treatment and then reduces the lime water concentration with each
successive treatment until the bleeding is under control.
Maintenance crews typically start with an average concentration on the order
of 5 sacks (250 pounds total) of hydrated lime in 1000 gallons of water. This
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PAVEMENTS
initial concentration can be as low as 4 sacks and as high as 8 sacks dependent upon the site factors such as environmental conditions and the severity of
bleeding. For example, 5 sacks might be appropriate for moderate bleeding
at an air temperature around 100°F ; whereas, 8 sacks might be appropriate
for very severe bleeding and air temperatures of 108°F or hotter.
In certain instances such as on a fairly remote low-volume FM road, crews will
typically be more aggressive with their initial concentrations and will use a
higher dosage than usual in an effort to minimize return trips.
The proper lime concentration for the initial treatment can be established/
fine-tuned by assessing how much lime powder is left on the road after drying
(how dusty it is). If lime powder is left on the pavement surface after drying,
the initial concentration should be cut back.
The rule of thumb for the second treatment and subsequent treatments is to
cut the initial concentration in half. For example, if the initial concentration
was 5 sacks, the second treatment concentration would be 3 sacks. In some
cases the bleeding might require more than two applications of lime water. In
that case, maintenance forces would probably use 3 sacks again. The lime
water treatment approach is subjective to some degree, and is based on past
experience and performance for the most part.
Recommended practice for applying lime water may be summarized as follows:
•
•
Apply lime water when pavement is hot for fast drying.
•
•
•
•
Never apply faster than 45 MPH.
•
•
If dust is excessive or cars are getting a white residue on them, cut
back lime in mixture (reduce concentration).
Pour lime in tank prior to adding water or while adding water.
Use goggles, gloves and dust mask.
Every other nozzle can be plugged and still get an effective coverage.
Tanks will need a method to circulate the mixture. This will keep the
lime suspended in the water.
Empty tank at the end of each day. The tank may need to be rinsed
out if there is a heavy build-up of lime in the bottom of the tank.
Page 59
Some debate exists relative to timing of the lime water treatments. The standard, customary practice recommended herein is reactive – that is, to treat
bleeding pavements in the afternoon when the pavement is hot. In contrast,
another philosophy is to apply lime water in the morning before the bleeding
starts (preemptive).
HELPFUL TIPS
Lime water treatments should begin as soon as the initial signs of bleeding
begin to appear. This will help prevent further deterioration and damage to
the pavement. Once bleeding starts and the seal coat or surface treatment
starts to peel up to the underlying layer or base, the extent of the problems
and damage can rapidly escalate; for example, grow from a 1/2 mile section
to a 10 mile section.
Some Districts include a general note in the seal coat or surface treatment
construction plans requiring the contractor to provide a water truck/lime water setup to cool any hot spots or to treat a bleeding asphalt pavement during
construction, if needed. This requirement enables the contractor to be ready
with the proper equipment and materials in the event of bleeding during construction.
Some maintenance personnel view lime water treatment as a secondary approach. If Grade 5 aggregate will stick to the pavement, use it first. If not,
lime water would be the next best thing.
CONCERNS
Some asphalts might not be amenable to treatment with lime water. Intuition
suggests that certain emulsions would fall in this category, especially on a
fresh seal, since emulsion is a water-based product. In questionable circumstances, a small test section is recommended to confirm the treatment will do
more good than harm.
One of the disadvantages of this treatment is that lime water puts a white film
on everything and kills the reflectability of pavement markings and striping.
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PAVEMENTS
Lime is corrosive to paint. If the public is allowed to drive through wet lime
slurry, this might result in claims by the public that lime has splattered their
vehicles and damaged the paint. Because lime water is caustic, splashing
potentially creates a safety concern for pedestrians/passers-by.
Lime water is considered to be a quick and cheap treatment method for
bleeding asphalt pavements, but results may be very short term in some circumstances.
Page 61
APPLY WATER TO COOL PAVEMENT SURFACE
DESCRIPTION
Water (not mixed with lime or other additives) is sometimes spray-applied to
cool off bleeding asphalt pavements.
APPLICATION
Both construction contractors and maintenance crews use this method. The
typical application is as an emergency, stop-gap treatment to arrest bleeding
on a freshly placed seal coat in order to minimize the chances of the seal
coat being picked up.
EFFECTIVENESS
Water is considered a temporary measure to cool the pavement and to buy
some time when other treatment methods or materials are unavailable. Water is considered to be a very short term solution for bleeding. Effectiveness
ranges from 30 minutes to a few hours. Multiple applications are normally
required.
MATERIALS
The water does not need to be potable. For example, brine water – which is a
byproduct of oilfield operations – has been used in west Texas.
PROCEDURE
Customary practice is to apply water using a standard construction water
truck equipped with a spray bar.
HELPFUL TIPS
Some maintenance supervisors prefer to use a single straight nozzle and let
the water splash directly from the truck on to the pavement surface. When
used, water spreader bars — such as a cement spreader bar with large openings that will not clog — are preferred.
This treatment method should only be used for small isolated areas.
CONCERNS
Some maintenance personnel believe that straight application of water is a
waste of money that might last 30 minutes at best. Its use should be limited
to construction if there is no other option available.
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PAVEMENTS
Water can conceivably make things worse with certain asphalts such as a
high float emulsions.
This treatment method can create more problems in certain instances. For
example, the pavement surface may get hotter and bleed more after the water evaporates.
Page 63
REMOVE BLEEDING PAVEMENT SURFACE AND
REPLACE WITH NEW SEAL COAT
DESCRIPTION
When a fresh seal coat has gone bad, it can be removed and replaced with a
new seal coat.
APPLICATION
This option is used only in the event of total failure of a new seal coat. The
problem seal coat is usually removed (scraped off) by a blade.
This is not considered a maintenance seal in that it is not done by maintenance personnel. The new seal coat is usually applied by a contractor in the
case of bleeding/failure during new construction. Otherwise, it would be done
by the district Special Jobs Crew.
EFFECTIVENESS
This treatment method is considered to be a long term solution to bleeding
pavement surfaces.
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PAVEMENTS
MATERIALS
Asphalt binder and aggregate are typical materials for seal coats and are
selected on a case-by-case basis. Materials should conform to TxDOT Standard Specifications.
PROCEDURE
Recommended seal coat practices and procedures, as defined in the TxDOT
Seal Coat Manual, should be used.
HELPFUL TIPS
The cause of bleeding should be determined prior to applying a new seal
coat.
CONCERNS
This treatment method is used on rare occasions in the event of total failure
of a new seal coat.
Page 65
OTHER SOLUTIONS—BLEEDING PAVEMENTS
OVERVIEW
Maintenance forces have used various other methods to treat bleeding pavement surfaces. These other methods are used infrequently but are mentioned since they may have limited application for treatment in a particular
circumstance. Following are brief descriptions of these less commonly-used
treatment methods specific to bleeding.
DRY POWDERED LIME
Powdered lime, purchased in bulk, can be dry-applied to a bleeding seal coat
or surface treatment. Both construction lime (hydrated lime, quicklime) and
agricultural lime (crushed limestone and chalk) have been used. The powdered lime is broadcast over the affected area using a shovel. Larger areas
are treated using a tailgate spreader.
PORTLAND CEMENT
Maintenance crews have used Portland cement to treat smaller areas of
bleeding. The powdered cement is broadcast over the affected area using a
shovel or a tailgate spreader.
MICRO-BLAZE®.
A propriety product called Micro-blaze® has been used to treat bleeding asphalt pavement. Commonly used by fire-fighters to treat chemical spills, Micro-blaze® is an emergency liquid spill control product consisting of a nontoxic formulation of biological activators and selected non-pathogenic microbes, which digest fats, oils and grease, protein, starches and odors caused
by organic waste. When applied to a hydrocarbon spill (e.g., liquid asphalt), it
will disperse the hydrocarbon on contact, inert the hydrocarbon so that it is
non-flammable, and will leave no slippery residue when washed down.
BRINE WATER Contractors have used “brine water” to treat bleeding asphalt pavements
during construction. This material is inexpensive, readily available and is a
by-product of the oil field. They believe it works like lime water. Brine water is
“salty, crusty, ugly & cheap.”
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PAVEMENTS
APPLY VERY CLEAN, HOT AGGREGATE
The idea behind this treatment is that the asphalt must be hot and soft
enough (liquid enough) to achieve adequate rock embedment into the asphalt to get the rock to stick. Application of hot aggregate enhances this
process. Clean seal-coat aggregate is heated at a hot mix plant, delivered to
the site, placed on the bleeding pavement surface, and rolled in using a steel
wheel roller.
Page 67
FLUSHING
Flushed asphalt pavement, in contrast to bleeding pavement, is typically not a
maintenance problem that must be addressed immediately. The maintenance thresholds that call for treatment of a flushed asphalt pavement include:
• The roadway surface is slick, with low skid resistance.
• The pavement surface is very slippery, particularly in wet weather conditions.
• The flushing is accompanied by rutting and water accumulation in the
wheel paths.
Flushed asphalt pavement problems are often found in the wheel paths. This
type of pavement distress does not normally warrant immediate or emergency maintenance measures; however, it must be monitored closely to minimize the probability for escalation into a wet weather safety concern or a
bleeding pavement problem.
The pavement surface in these areas is usually slick and characterized by a
loss of traction, low skid numbers, and a higher potential for hydroplaning
during wet weather. Incidents including vehicles sliding off of the roadway
and wet weather accidents can trigger the need for treatment.
Maintenance forces have employed a variety of methods to treat flushed
asphalt pavements. The basic approaches are: (a) to retexture the existing
flushed pavement surface, or (b) to add a new textured surface over the
flushed pavement. The method chosen often depends upon economics as
well as the availability of materials, manpower and equipment at the time of
treatment.
the flushing problem as well as many other factors including environmental
conditions (temperature and humidity), type of roadway, traffic levels and
types, specific locations on a roadway (curves, intersections, urban or rural
environments) and the like. Practical wisdom is the key to success of any
treatment solution.
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SOLUTIONS FOR FLUSHED ASPHALT
PAVEMENTS
The objective of treatment is to increase the pavement macrotexture and
improve skid resistance. In instances where rutting and flushing are seen in
the wheel paths, maintenance crews will also try to improve surface water
drainage flow off of the roadway, especially away from (or out of) the wheel
paths.
In contrast to treatment of bleeding pavement surfaces, the timing for the
retexturing options is during cooler weather when the asphalt binder is least
active. The time for new texturing options varies depending on the method.
The following table summarizes maintenance solutions for treatment of
flushed pavement surfaces.
Maintenance
Solutions for Flushed
Asphalt Pavements
Solution Type
Add New
Textured
Surface
Cold milling to remove
flushed asphalt
Apply new seal coat
Microsurfacing
Thin asphaltic concrete
overlay
■
Short
Term
Mid Term
Cost
Long
Term
■
Low
Medium
■
■
High
■
■
■
■
■
■
■
■
■
■
■
■
■
■
Ultra high pressure water
cutting
Other solutions
Retexture
Existing
Pavement
Effectiveness
■
■
■
■
■
■
■
■
Page 69
COLD MILLING TO REMOVE FLUSHED ASPHALT
WITH/ WITHOUT REPLACEMENT
DESCRIPTION
Cold milling is the controlled removal of the surface of the existing pavement
to the desired depth, with specially designed milling equipment. Milling is
done either to prepare the surface to receive overlays (by removing rutting,
flushing, and surface irregularities), to restore the pavement cross slopes and
profile, or to re-establish the pavement’s surface friction characteristics. Surface removal is typically done with a milling machine which uses a drum
equipped with carbide-tipped teeth that impact and chip the pavement surface. The resulting textured pavement can be used immediately as a driving
surface.
APPLICATION
A typical application of milling is to reshape/retexture flushed pavement surfaces that exhibit rutting and low skid resistance. Maintenance personnel
use milling to:
•
Remove excess asphalt (in a solid state) from a flushed or slick
pavement.
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PAVEMENTS
•
Minimize or eliminate the potential for a flushed pavement to bleed
in hot weather.
•
Restore or establish pavement texture and skid resistance/traction.
The typical application is where rutting has occurred in the wheel paths, causing water to accumulate during rains, and skid resistance and hydroplaning
are a concern.
Milling is also used quite often at intersections where heavy starting and stopping movements have caused rippled, bumped and otherwise rough pavement surfaces.
This treatment method is commonly used as part of a mill and inlay operation
where an old section of pavement is removed and replaced with a new asphaltic concrete overlay.
EFFECTIVENESS
Cold milling is considered to be a long term treatment for the restoration of
pavement texture (skid resistance) associated with flushed pavements. This
treatment is not considered effective for treatment of actively bleeding pavement surfaces.
MATERIALS
The typical equipment requirements for cold milling include:
•
modern, self-propelled cold milling machine
•
haul trucks
•
water truck
•
sweeper or power broom
•
traffic control
Milling equipment is available in a variety of sizes, ranging from mini-milling
machines for localized milling, to high capacity machines capable of milling
full lane widths in one pass.
PROCEDURE
Milling is typically done in the wheel paths in cooler weather when the asphalt
is in a solid state and can be cut more efficiently. Milling may be done either
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COLD MILLING TO REMOVE FLUSHED ASPHALT
WITH/ WITHOUT REPLACEMENT, CONT’D.
by maintenance forces or by contract under a Maintenance Service Agreement.
Maintenance forces typically perform “spot milling” operations with a milling
attachment on a skid steer loader (Bobcat) with a drum width of approximately 18 to 24 inches. Milling can be to any depth; however, typical cuts for
treatment of flushed pavements range from 1/2 to 3/4 inch maximum. Two
or more passes may be required.
The miller essentially restores a planar surface in the wheel paths thus eliminating the troughs that tend to hold water during rains which can create a
safety hazard associated with hydroplaning. The depth, surface inclination
and width of the cut are apparently adjustable to some degree. In the process of milling these flushed areas, layers of excess asphalt may also be removed.
Contractors typically utilize larger, higher production machinery which can be
equipped with drum widths ranging from 4 to 12.5 feet. Contract equipment
can mill an entire lane in one pass and load the spoils (reclaimed asphalt
pavement - RAP) by conveyor belt into a dump truck at the same time. The
contractor typically provides the milling machine with an operator and one
helper.
After the milling operation is completed, the pavement has a rougher texture
and is grooved along the longitudinal axis of the roadway. This may garner
some complaints from the traveling public – just as some seal coats with
Grade 3 rock do – however, traveler safety is enhanced.
The pavement surface is often left in a milled condition for extended periods
of time until subsequent treatments such as a new seal coat or hot mix overlay are applied. In some cases, maintenance crews will go back over the
milled areas with premix patch materials using blade level-up methods in
advance of subsequent seal coat or overlay operations.
HELPFUL TIPS
Milling to remove excess asphalt and restore texture to flushed pavements is
normally done in the winter during cool or cold weather. Milling cannot be
done when the asphalt is “live.”
Some district maintenance personnel require skid resistance measurements
in any areas of concern prior to planning corrective measures.
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PAVEMENTS
Milling leaves the surface of the asphalt open and more susceptible to water
penetration. In some cases, maintenance personnel will shoot an underseal
or a fog seal, or place a thin overlay over the affected area.
CONCERNS
One downside of milling is that it has a tendency to remove the positive benefit of a surface seal. Eventually it will be necessary to apply another seal coat
or construct a hot mix overlay to effectively seal the pavement from the intrusion of water.
Milling can sometimes make a rutting situation worse.
Certain seal coat materials have a tendency to gum up the milling machine in
hot weather, especially the crumb rubber seal.
Page 73
NEW SEAL COAT
DESCRIPTION
A new seal coat, typically in the form of a strip seal or spot seal, may be applied over a flushed pavement surface to arrest the flushing and restore skid
resistance.
APPLICATION
The ideal application for this option is to treat the wheel paths of a flushed
pavement where there is also minor rutting and loss of skid resistance.
On a much larger scale, this type of treatment is used to treat aged, flushed
pavements where the flushing occurs due to aggregate wear and abrasion.
This is normally accomplished as part of the district seal coat program. This
would be a full-width repair.
EFFECTIVENESS
This treatment method seals the roadway surface and improves skid resistance. It is considered to be a long term solution to flushed pavement surfaces.
MATERIALS
Asphalt binder and aggregate for new strip/spot seals are typical materials
for seal coats and are selected on a case-by-case basis. Materials should
conform to TxDOT Standard Specifications.
PROCEDURE
Recommended seal coat practices and procedures, as defined in the TxDOT
Seal Coat Manual, should be used.
New seal coat materials should be similar to those used for the original seal
coat.
When the flushing is severe, the asphalt binder application rate should be
lighter. For typical applications, the amount of binder should be reduced by,
say, 20 to 30 percent.
HELPFUL TIPS
The cause of flushing should be determined prior to applying a new seal coat.
Patches or soft areas of the pavement substrate should be corrected before
applying the new seal coat.
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PAVEMENTS
CONCERNS
Some maintenance personnel prefer not to apply a new seal coat (strip and
spot seal) over a flushed pavement because it adds more asphalt and has
the potential to make the condition worse.
This method should not be used to treat a bleeding pavement surface. The
concern is too much asphalt. Bleeding will normally come through a new seal
coat if it is not treated in advance of seal coat operations.
Some claim that the same concern exists relative to flushing. That is, preexisting flushing may presumably reappear through the new seal coat, especially in the wheel paths.
Page 75
MICROSURFACING
DESCRIPTION
Microsurfacing is a tough, durable, thin cold overlay material which has been
used for both corrective and preventive maintenance to restore the original
properties to structurally sound pavements. By definition, microsurfacing is a
mixture of polymer modified asphalt emulsion, mineral aggregate, mineral
filler, water, and other additives. These components are mixed together at
the jobsite in a pugmill that is mounted on a self-propelled hauling unit.
APPLICATION
Microsurfacing is used on a limited basis to remediate problems associated
with minor rutting, flushed pavement, ride quality and skid resistance (usually
a combination of these).
Selection of the microsurfacing method typically focuses on correcting problems associated with rutting in the wheel paths and providing improved
macrotexture and skid resistance on a roadway. Flushing problems are often
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PAVEMENTS
addressed as a by product of this treatment. Microsurfacing is not typically
used in and of itself to correct a flushed pavement problem. It is also not
used to mitigate bleeding.
Microsurfacing is normally done on long stretches of road, often in urban
environments, for roadways with high traffic volumes. The pavement section
must be structurally sound.
Microsurfacing has also been utilized to restore skid resistance in intersections where the pavement is flushed, typically due to polishing (wearing down)
of the aggregate.
Microsurfacing should not be used when a seal coat is needed – it is not a
sealer. It is sometimes used as an emergency treatment for skid repair. This
treatment is considered to be the next level up from a new seal coat.
EFFECTIVENESS
Microsurfacing is considered to be a long term treatment for flushed pavements and has to be done by specialty contractors under an engineer’s contract. Maintenance personnel indicate that they have used this solution successfully and can get about 5 to 7 years of service life.
MATERIALS
This method consists of furnishing and placing a microsurfacing system which
is a mixture of cationic polymer-modified asphalt emulsion, mineral aggregate, mineral filler, water, and other additives. Microsurfacing materials,
practices and procedures should conform to TxDOT Standard Specifications,
Item 350.
PROCEDURE
Microsurfacing is made and applied to existing pavements by a specialized
machine, which carries all components, mixes them on site, and spreads the
mixture onto the road surface.
Materials are continuously and accurately measured, and then thoroughly
combined in the microsurfacing machine’s mixer. As the machine moves
forward, the mixture is continuously fed into a full-width surfacing box which
spreads the microsurfacing across the width of the traffic lane in a single
pass. Specially engineered rut boxes may also be used.
Page 77
MICROSURFACING, CONT’D.
The new surface is initially a dark brown color and changes to the finished
black surface as the surface cures and is opened to traffic.
HELPFUL TIPS
Microsurfacing is done by specialty contractors and normally takes about a
year to bring to fruition – from initial planning to construction.
Microsurfacing must be used on the “right road.” The existing pavement
must be structurally sound, preferably without cracking, and cannot flex.
Falling weight deflectometer measurements are usually made on the existing
pavement to provide the necessary data for the evaluation of the pavement
section.
Microsurfacing is used to mitigate minor rutting and flushing, improve ride
quality and restore skid resistance. Shallow ruts can be filled with a “scratch
course” (one of two courses). The final treatment thickness is usually about
3/4 inch.
The microsurfacing should be constructed in the fall, as opposed to the heat
of the summer.
The microsurfacing can be seal coated after it has served its useful life.
CONCERNS
Some maintenance personnel will not use microsurfacing over a flushed asphalt pavement due to concerns with the excess asphalt working its way
through the surface treatment. That is, the microsurface does not adequately
resist the migration of excess asphalt (flushing, not bleeding) through the
treatment.
The microsurfacing material contains cement and can be prone to cracking
with excessive pavement deflections.
Another downside of this treatment method is the tendency for cracking in
the existing pavement to reflect through the overlying microsurfacing. It is not
unusual to see some cracking in the treatment during its service life.
This proprietary treatment method is relatively expensive.
Page 78
PAVEMENTS
Page 79
THIN ASPHALTIC CONCRETE OVERLAY
DESCRIPTION
Maintenance forces use blade level-up techniques to create thin asphaltic
concrete overlays in order to remediate chronic problems with flushed pavement, typically in association with repair of rutting, patches, and other pavement defects. Thin overlays are also sometimes constructed using asphalt
laydown machines. This is usually done in conjunction with seal coat preparation work in advance of seal coat operations.
APPLICATION
Thin overlays are often used to treat flushed wheel paths with minor rutting,
and as a wearing course over a milled pavement. Thin overlays normally are
not used to treat a flushed pavement alone; that is, a pavement without any
other problems.
Relative to a flushed pavement, the purpose of blade level-ups and other thin
asphaltic concrete overlays is to restore friction and skid resistance. In rare
circumstances thin overlays have been applied where minor bleeding is present, but thin overlays are typically not used over bleeding asphalt pavements
due to concerns with propagation of the asphalt through the overlay.
Thin overlays frequently are applied in high-risk areas such as where accidents have occurred, along curves, and in the most critical (slick) pavement
sections. Frequent complaints about hydroplaning or vehicles running off the
road over larger areas or more continuous sections of roadway normally move
the affected road up the overlay candidate list.
Mill and fill operations, where the overlay is constructed with a laydown machine, may be used at intersections where start-stop and other abusive traffic
movements occur. This type of overlay is considered as the final, last-resort
type of treatment where no other solution seems to work.
EFFECTIVENESS
Thin asphaltic concrete overlays are one of the more expensive solutions for
flushed pavements, but provide a long-term service life.
MATERIALS
Maintenance forces use various materials to construct thin asphaltic concrete overlays.
Page 80
PAVEMENTS
When using blade level-up techniques, Limestone Rock Asphalt (LRA) premix
(Type FS or DS) is the most commonly applied material. Maintenance crews
prefer to use LRA premix because it is easy work. LRA premix with trap rock
is particularly desirable in that it provides good traction due to the presence
of the very hard, abrasion-resistant trap rock material.
Hot mix cold laid (HMCL) can be used without a tack coat if sufficient asphalt
is present on the pavement.
Maintenance forces have used Type F, Type D, and Type C hot mix for thin
overlay applications on flushed pavements. These materials can be delivered
directly to a site from the hot mix plant, are easily compactable, and will not
absorb as much seal coat oil as the LRA. Typically hot mix is applied using
the laydown machine.
Page 81
THIN ASPHALTIC CONCRETE OVERLAY,
CONT’D.
Asphaltic concrete materials should conform to TxDOT Standard Specifications.
PROCEDURE
Maintenance forces apply thin overlays using both blade levelup techniques
and using asphalt laydown machines.
A tack coat is frequently applied beneath thin overlays/ patches. In areas
where the existing pavement is rich in asphalt, it is possible to lighten or in
some cases eliminate the tack coat. Thin hot mix overlays should be rolled
(compacted) with a steel wheel or pneumatic tired roller.
Recommended overlay practices and procedures, as defined in the TxDOT
Hot Mix Manual, should be used.
Page 82
PAVEMENTS
HELPFUL TIPS
Older (dry) LRA premix material has been used successfully to soak up excess
asphalt on bleeding pavements. Water may be added to the LRA if it is too
dry.
For reasons of cost and mobilization effort, overlays are typically used to treat
larger areas of pavement.
CONCERNS
This solution is normally used as a last resort on a bad stretch of road when
other remedies have failed and conditions warrant this level of correction.
Some maintenance personnel believe that LRA is too porous, it absorbs too
much seal coat oil, is difficult to compact and achieve good density, and
needs to be compacted by the maintenance crews (rather than traffic), something that is rarely done.
Some maintenance personnel believe that blade patches create more problems than they solve at times. Ultimately the patches have to be sealed.
Some maintenance personnel do not use thin HMAC overlays to mitigate
problems with bleeding and flushing, but instead use overlays to correct pavement structure problems.
Page 83
ULTRA HIGH PRESSURE WATER CUTTING TO
REMOVE EXCESS ASPHALT AND RESTORE
TEXTURE
DESCRIPTION
The ultra high pressure (UHP) watercutter is an emerging technology, currently used in Australia/New Zealand and being evaluated by TxDOT for implementation in Texas, that holds promise for treatment of flushed pavements.
The UHP watercutter machine combines both watercutting and road cleaning
technologies in a single process to simultaneously remove excess binder and
contaminants from pavement surfaces, and retexture aggregate surfaces
improving road surface macrotexture and aggregate microtexture. The UHP
watercutting machine was designed and fabricated in New Zealand by Fulton
Hogan Limited.
Page 84
PAVEMENTS
The UHP watercutter combines a truck-mounted UHP pump, water supply,
and vacuum recovery system with an independently operated umbilical deckblaster. A rotating spraybar uses specialized nozzles to direct very fine jets of
ultra-high pressure water (36,000 psi) at ultrasonic velocity (mach 1.5) on to
the road surface.
APPLICATION
High pressure water can be used to restore surface texture and skid resistance on all types of pavement surfaces that have become slick or flushed, by
removing excess binder, oil, grease or rubber tire particles. It can also be
used to remove pavement markings, striping, spills, etc.
This treatment can be used in advance of seal coating operations to treat
asphalt-rich patches, minor bleeding problems, and flushed areas and to
create a uniform surface texture for a subsequent (new) seal coat.
This treatment is less suitable for treating thin surfaces which may be easily
damaged or dislodged.
EFFECTIVENESS
Life expectancy of the treatment will be influenced by, among other things,
the underlying cause of asphalt flushing and the likelihood of further aggregate embedment or flushing of binder.
Promotional literature for this treatment option claims that when the UHP
watercutter is used to retexture a flushed pavement, several years should
elapse before further retexturing or resurfacing is required (data from New
Zealand). Effectiveness has not been established for U.S. (Texas) roadway
conditions.
Retexturing removes the excess build-up of asphalt binder that would otherwise make further sprayed seal treatments difficult to apply without the risk
of a repeat of flushing/bleeding in the new seal.
Cost of the treatment will depend on the size of the project, with larger machines treating more surface area in a single shift.
MATERIALS
The UHP watercutter equipment consists of very high pressure pumps, usually truck mounted and self contained, and applicators, which may vary from
the hydro-mower (umbilical) type for treating smaller areas, to large tractor or
Page 85
TEXTURE, CONT’D.
truck mounted units. Both machine types include tanks for the supply of
fresh water and storage of collected water and debris. Machines currently are
in use in Australia and New Zealand.
Precise control of pressure, water volume and speed allows effective removal
of excess asphalt binder and surface contamination with minimal damage to
the surface or dislodgement of coarse aggregate particles. Powerful suction
heads are used to collect water and debris from the surface for later disposal.
PROCEDURE
Detailed procedures for UHP watercutting in Texas have not been developed.
The following general requirements and procedures are typical for the UHP
watercutting process:
•
A source of clean water is required. Water pressure needs to be controlled to prevent damage to the surfacing. The hardness of the binder
in a seal or asphalt will influence the pressure required and time taken
to achieve a satisfactory result.
•
The process leaves a slimy residue which must be washed off. State and
local environmental requirements regarding disposal of the water and
any debris must be followed.
•
The area will need traffic control to prevent damage to vehicles or skidding problems.
HELPFUL TIPS
The process is most effective on sprayed seals and asphalt showing loss of
texture due to flushed binder. Excess surface binder is removed from a
flushed sprayed seal to expose the well-textured aggregate surface.
UHP watercutter treatments should be done during the cooler time of the year
when pavement temperatures are lower and the asphalt material is stiffer
(more brittle) and can be cut more efficiently and effectively achieving better
production rates.
The process can be operated in cold, damp or wintry conditions where other
resurfacing options are not feasible. Treatments can also be made during wet
weather and at night if necessary.
Page 86
PAVEMENTS
Treatment of flushed asphalt pavements should not be done during the hotter times of the year when the pavement temperatures are higher and the
asphalt is more ductile (softer). Watercutting during these conditions will
significantly reduce production rates and treatment effectiveness, and has a
tendency to “gum up” the machinery requiring continual cleaning and maintenance of the equipment.
Retexturing should not be used on thin seals where rapid failure may occur as
a result of insufficient remaining binder to adequately hold the seal in place.
Care must also be applied when treating very weak and previously patched
pavements. Contingency plans may be needed for restoration of patches
damaged by the retexturing operations.
Page 87
TEXTURE, CONT’D.
CONCERNS
Waste materials must be disposed of in an approved environmental manner.
Depending on the process, a typical day’s operation may collect up to 2-4
cubic yards of solids and use up to 10,000 gallons of water.
Disposal is covered by state environmental regulations. Waste disposal classifications must be determined before commencing work. Classification of liquid wastes and solid wastes that may liberate free liquids when stockpiled,
transported and disposed of, are particularly important. All stockpiled waste
materials must have environmental controls in place to avoid pollution of the
surrounding environment.
Page 88
PAVEMENTS
Page 89
OTHER SOLUTIONS—FLUSHED PAVEMENTS
OVERVIEW
Maintenance forces have used various other methods to treat flushed pavement surfaces. These other methods are used infrequently, but are mentioned since they may have limited application for treatment in a particular
circumstance. Following are brief descriptions of these less commonly-used
treatment methods.
GROOVED MOLD BOARDS
Maintenance personnel have cut grooves in a motor grader mold board in
order to score the flushed pavement surface. This treatment improves texture (skid resistance/traction) on the pavement surface.
A similar approach consists of welding carbide bullet tips on the mold board.
Either approach creates a tool which can be used to score/texture the pavement surface.
Page 90
PAVEMENTS
HEAT PAVEMENT SURFACE AND ROLL IN HOT
AGGREGATE
In this method, the flushed pavement surface is pre-heated and new aggregate is applied, the idea being to get the asphalt hot and soft enough to stick
the aggregate to the pavement surface. Maintenance forces can use a weedburner, either hand-wand or tractor-mounted, or a more sophisticated heaterplaner, to preheat the pavement surface. Preheating is followed by the application of a hot aggregate (typically, Grade 4) which may be obtained from a
hot mix plant or may be preheated in the drier of a portable patching machine. The hot aggregate is rolled in using a steel wheel roller.
SLURRY SEAL
Slurry seals, which are similar to microsurfacing, are mixtures of cationic
emulsified asphalt, mineral aggregates, mineral filler, water and additives,
properly proportioned, mixed and spread with a machine over a properly prepared surface. A slurry mixture forms an impervious thin overlay over an existing pavement. The aggregate is relatively fine (less than1/4 inch). The fine
aggregate at the pavement surface corrects loss of skid resistance typically
associated with flushed pavement by providing increased friction. The slurry
seal is a thin surface treatment intended to provide pavement sealing and
some pavement texturing, but is not appropriate to resolve pavement structural deficiencies. Like microsurfacing, slurry seals are applied only by specialty contractors.
Page 91
INTERSECTIONS
Seal coats and surface treatments have limitations in their ability to resist the
effects of heavy traffic and abusive traffic maneuvers at high-stress traffic
areas such as intersections, median openings, and the like.
Turning and braking of heavy vehicles can cause aggregate to roll, leading to
loss of aggregate and bleeding of the seal. Concentrations of heavy traffic at
intersections may cause aggregate embedment, leading to flushing of the
binder.
The maintenance thresholds that call for treatment of bleeding and flushed
asphalt pavements at intersections include:
• The asphalt begins to soften and liquefy on the roadway surface.
• Turning movements cause the seal coat/surface treatment aggregate to
dislodge (shelling).
• Turning movements cause the seal coat/surface treatment aggregate to
mound up/shove.
• The roadway surface is slick and slippery, with low skid resistance, particularly in wet weather conditions.
Fundamentally, the problem with intersections and other high-stress traffic
areas is a mismatch between structural/performance requirements and the
pavement surface type.
Maintenance forces have employed a variety of methods to treat bleeding
and flushed asphalt pavements at high-stress traffic areas such as intersections. The basic approaches are (a) to retexture the existing pavement surface, or (b) to replace the seal coat /surface treatment with a new, more durable, pavement material. The method chosen depends upon economics as
well as the availability of materials, manpower, and equipment at the time of
treatment.
the problems as well as many other factors including environmental conditions, type of roadway, traffic levels and types, specific locations on a roadway
Page 92
SOLUTIONS FOR BLEEDING/FLUSHED
PAVEMENTS AT INTERSECTIONS
(curves, intersections, urban environments) and more. Practical wisdom is the
key to success of any treatment solution.
The objective of treatment at intersections is to provide a pavement surface
which is less susceptible to damage due to high traffic and abusive traffic
maneuvers. This includes not only correction of bleeding/flushing but also
rutting, shoving, and other roadway defects. Corrective action should also try
to improve surface water drainage flow off of the roadway, especially away
from (or out of) the wheel paths.
The following table summarizes potential solutions for treatment of bleeding
and flushed pavements at intersections.
Maintenance Solutions
for Bleeding/Flushed
Asphalt Pavement at
Intersections
Microsurfacing
Solution Type
New
Textured
Surface
Durable
Overlay
Effectiveness
Short
Term
Mid
Term
■
Cost
Long
Term
Low
■
Medium
High
■
Hot mix asphalt pavement
■
■
■
Portland cement
concrete pavement
■
■
■
Racked-in seal
■
■
■
Arguably the first three solutions — microsurfacing, hot mix asphalt, and portland cement concrete — are not routine maintenance activities. These have
been included to illustrate the range of options and for the sake of completeness.
In contrast, the fourth solution — the racked-in seal — is very compatible with
typical maintenance activity and may prove to be a promising alternative for
treatment of bleeding/flushing at intersections.
Page 93
MICROSURFACING
DESCRIPTION
Microsurfacing is a tough, durable, thin cold overlay material which has been
used for both corrective and preventive maintenance to restore the original
properties to structurally sound pavements. By definition, microsurfacing is a
mixture of polymer modified asphalt emulsion, mineral aggregate, mineral
filler, water, and other additives. These components are mixed together at
the jobsite in a pugmill that is mounted on a self-propelled hauling unit.
APPLICATION
Microsurfacing is used to remediate problems associated with minor rutting,
flushed pavement, ride quality and skid resistance (usually a combination of
these). Microsurfacing is not used to correct bleeding.
Selection of the microsurfacing method typically focuses on correcting problems associated with rutting in the wheel paths and providing improved
macrotexture and skid resistance on a roadway. Relative to intersections,
microsurfacing provides a tough, durable pavement surface which is less
susceptible to damage from high traffic and abusive traffic maneuvers.
Microsurfacing is often done in urban environments for roadways with high
traffic volumes. The pavement section must be structurally sound.
EFFECTIVENESS
Microsurfacing is considered to be a long term treatment for flushed pavements and has to be done by specialty contractors. Maintenance personnel
indicate that they have used this solution successfully and can get about 5 to
7 years of service life.
MATERIALS
This method is done under construction contract and consists of furnishing
and placing a microsurfacing system consisting of a mixture of cationic polymer-modified asphalt emulsion, mineral aggregate, mineral filler, water, and
other additives. Microsurfacing should conform to TxDOT Standard Specifications, Item 350.
PROCEDURE
Microsurfacing is made and applied to existing pavements by a specialized
machine, which carries all components, mixes them on site, and spreads the
mixture onto the road surface.
Page 94
Materials are continuously and accurately measured, and then thoroughly
combined in the microsurfacing machine’s mixer. As the machine moves
forward, the mixture is continuously fed into a full-width surfacing box which
spreads the microsurfacing across the width of the traffic lane in a single
pass. Specially engineered rut boxes may also be used.
HELPFUL TIPS
Microsurfacing is done by specialty contractors and normally takes about a
year to bring to fruition – from initial planning to construction.
Microsurfacing must be used on the “right road.” The existing pavement
must be structurally sound, preferably without cracking, and cannot flex.
Falling weight deflectometer measurements are usually made on the existing
pavement to provide the necessary data for the evaluation of the pavement
section.
CONCERNS
Some maintenance personnel will not use microsurfacing over a flushed seal
coat or surface treatment due to concerns with the excess asphalt working its
way through the surface treatment. That is, the microsurface does not adequately resist the migration of excess asphalt (flushing) through the treatment.
The microsurfacing material contains cement and can be prone to cracking
with excessive pavement deflections.
Another downside of this treatment method is the tendency for cracking in
the existing pavement to reflect through the overlying microsurfacing. It is not
unusual to see some cracking in the treatment during its service life.
This proprietary treatment method is relatively expensive.
Page 95
HOT MIX ASPHALT PAVEMENT
DESCRIPTION
Hot mix asphalt pavement may be used to remediate chronic problems with
rock loss and attendant bleeding/flushing at intersections, normally in association with repair of rutting, patches, and other pavement defects. The construction process will typically be a mill and fill operation. The existing pavement surface (seal coat/surface treatment) is removed to a predetermined
depth and hot mix asphalt is placed with a laydown machine.
APPLICATION
The decision to use hot mix asphalt at an intersection largely depends on
economic considerations including life cycle costs. Among other things, this
will be a function of the existing pavement condition, traffic volume, traffic
type, and long term maintenance objectives.
Frequent maintenance repair effort and/or consumer complaints and concerns about safety move the affected intersection up the candidate list.
EFFECTIVENESS
Hot mix asphalt is one of the more expensive solutions for bleeding/flushed
pavement at intersections, but at the same time provides a long service life.
MATERIALS
Asphalt binder and aggregate for new hot mix pavements are typical roadway
construction materials and are selected on a case-by-case basis. Materials
should conform to TxDOT Standard Specifications.
PROCEDURE
Design of hot mix asphalt pavements for intersections must recognize that
slow-moving or standing loads subject the pavement to higher stress conditions, which may be enough to induce rutting and shoving. In addition, an
increase in trucks and heavier wheel loads can also play a significant role in
the premature failure of some pavements. Thus the pavement must be designed and constructed to withstand the more severe conditions.
To perform well, an intersection pavement must have adequate thickness to
provide the structural capacity to meet traffic needs. Pavement thickness
design must account for normal factors such as subgrade strength, drainage
and traffic. Any failed or weak layers in the existing pavement structure must
be removed since paving over existing failed material will likely result in recurring failure.
Page 96
Careful selection of the asphalt binder is key to providing desirable performance. A more rutting-resistant binder is needed at intersections. Aggregates
used in intersection mixtures must also be carefully selected. The aggregate
structure has to be capable of carrying the load and developing a high degree
of stone- to-stone interlock that will resist shearing.
The goal of the mix design process is to select and proportion appropriate
materials that resist rutting.
HELPFUL TIPS
The performance history of intersections and other similar high stress areas
should be evaluated. This will play a key role in deciding whether changes
should be made to the normal design and construction procedures.
CONCERNS
This method is among the more expensive treatment solutions.
Page 97
PORTLAND CEMENT CONCRETE PAVEMENT
DESCRIPTION
Portland cement concrete pavement may be used to remediate chronic problems with bleeding/flushing at intersections, normally in association with
repair of rutting, patches, and other pavement defects. The construction
process will typically be a mill and fill operation. The existing pavement surface (seal coat/surface treatment) is removed to a predetermined depth and
a full-depth Portland cement concrete pavement is placed.
APPLICATION
The decision to use portland cement concrete at a selected intersection
largely depends on economic considerations including life cycle costs. Among
other things, this will be a function of the existing pavement condition, traffic
volume, traffic type, and long term maintenance objectives.
Candidate intersections include those that are severely rutted and distressed
from loads, slow moving vehicles, and warm temperatures. Frequent maintenance repair effort and/or consumer complaints and concerns about safety
move the affected intersection up the candidate list.
EFFECTIVENESS
Portland cement concrete is particularly effective at handling the very rigorous stopping, starting, standing, and turning actions of vehicles at intersections.
Portland cement concrete provides a long term service life and requires very
minimal, if any future rehabilitation. The construction user costs and disruption to traffic that are necessary with future asphaltic concrete inlays during
its design life are reduced when portland cement concrete is used.
Proponents further claim that portland cement concrete offers a safer, more
durable, smoother, and longer-lasting pavement.
This is one of the more expensive solutions for bleeding/flushed pavements.
However, life cycle cost analysis of portland cement concrete reconstruction
versus asphalt concrete pavement reconstruction and future inlays shows
that portland cement concrete intersection reconstruction may compete with
asphalt paving.
MATERIALS
Portland cement concrete pavement materials should conform to TxDOT
Standard Specifications.
Page 98
PROCEDURE
Design of portland cement concrete pavements for intersections must recognize that slow moving or standing loads subject the pavement to higher stress
conditions. In addition, the increase in the number of trucks and heavier
wheel loads can also play a significant role in the premature failure of some
pavements. Thus the pavement must be designed and constructed to withstand the more severe conditions.
To perform well, an intersection pavement must have adequate thickness to
provide the structural capacity to meet traffic needs. Pavement thickness
design must account for normal factors such as subgrade strength, drainage
and traffic. Any failed or weak layers in the existing pavement structure must
be removed since paving over existing failed material will likely result in recurring failure.
HELPFUL TIPS
The performance history of intersections and other similar high stress areas
should be evaluated. This will play a key role in deciding whether changes
should be made to the normal design and construction procedures.
CONCERNS
The major disadvantage with portland cement concrete intersections is the
higher initial construction cost. This method is among the more expensive
treatment solutions.
Page 99
RACKED-IN SEAL
DESCRIPTION
The racked-in seal is a variation to the single course seal coat. The racked-in
seal consists of one heavy layer of binder followed by two layers of cover aggregate. The second layer of aggregate, or scatter coat, is smaller than (onethird to one-half size) the first layer. The smaller aggregate fills the voids and
displaces the binder further upward on the larger stone, thereby mechanically
locking the larger aggregate in position and producing a stable matrix.
APPLICATION
The racked-in seal has been proposed for use in Texas at high stress areas,
for example, heavily-trafficked intersections that otherwise would be sealed
with a traditional seal coat. It is frequently used in association with asphalt
emulsion binders.
The purpose behind the racked-in seal is to provide a stronger seal coat treatment. The scatter coat aggregate (rack rock) is intended to prevent the seal
Page 100
coat rock from sliding, rolling over or scrubbing off of the underlying pavement. The analogy is to billiards, where a rack holds cue balls in a tight pattern.
EFFECTIVENESS
The racked-in seal has been used in Europe, Australia and New Zealand on
roadways where traffic is heavy and/or fast.
The racked-in seal has been introduced in Texas only recently and has seen
limited application on system roadways. Recommended details for construction and performance data on Texas roadways are not available at this time.
MATERIALS
Asphalt binder, coarse aggregate, and fine aggregate for the racked-in seal
are typical materials for seal coats and are selected on a case-by-case basis.
Materials should conform to TxDOT Standard Specifications.
PROCEDURE
The racked-in technique involves a light application of a small size aggregate
(rack rock) over a traditional coarser aggregate seal coat. The rack rock is
applied before traffic and serves to reduce rolling over of coarse aggregate
particles during the critical initial aggregate reorientation stages of seal coat
compaction.
Generally the second application of aggregate (rack rock) is half the size of
the first. A typical combination is 0.2-inch rack rock (Grade 5) over 3/8-inch
seal coat rock (Grade 3). This enables the smaller aggregate to lodge in the
void spaces in the larger aggregate, holding the larger aggregate in place and
providing a strong mechanical key against traffic shearing forces.
Layout for a typical intersection would involve placing the racked-in seal at
least 100 feet beyond the intersection in all directions. Seal coat construction could use either hot asphalts or emulsions.
Two chip spreaders are required to construct the racked-in seal. The initial
portion of the racked-in seal is constructed like a normal seal coat. However,
the seal coat aggregate application rate is normally reduced so that the second layer of aggregate (rack rock) is firmly held as a permanent part of the
seal.
Page 101
RACKED-IN SEAL, CONT’D.
Rolling procedures are established in accordance with the characteristics of
the seal coating system. Intermediate rolling is advisable for the initial embedment of the seal coat rock forming the lower layer of a racked-in system.
A variation on the racked-in seal is the “dry lock” process. The terms “rackedin” and “dry lock” are used somewhat interchangeably. However, in the “dry
lock” technique the initial aggregate application rate (seal coat rock) is unchanged and it is expected that most of the second layer of aggregate (the
scatter coat, or rack rock) will be lost during the early service life of the seal.
HELPFUL TIPS
A Special Specification has been developed for the racked-in seal: SPECIAL
SPECIFICATION 3057, “Racked in Aggregate for Surface Treatments.”
Page 102
CONCERNS
Aggregate whip-off for the scatter coat (rack rock) should be anticipated for
the racked-in system.
The racked-in seal has been introduced in Texas only recently and has seen
limited application on system roadways. Recommended details for construction and performance data on Texas roadways are not available at this time.
Page 103
Page 104
Page 105
Page 106
Page 107
sponsored by the
Texas Department of Transportation
in cooperation with
U.S. Department of Transportation
Federal Highway Administration
November 2006
TECHMRT
MULTIDISCIPLINARY RESEARCH IN TRANSPORTATION
TEXAS TECH UNIVERSITY
Box 41023
Lubbock, TX 79409-1023
Phone: 806.742.3505
Fax: 806.742.4168
Web: http://www.techmrt.ttu.edu/

maintenance solutions for bleeding and flushed pavements

Transcription

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